Passive cooling options in combination with vector proofing for indoor heat reduction and mosquito control: a pilot study in rural western Kenya

doi:10.21203/rs.3.rs-4870484/v1

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Passive cooling options in combination with vector proofing for indoor heat reduction and mosquito control: a pilot study in rural western Kenya

https://doi.org/10.21203/rs.3.rs-4870484/v1

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Background

The rapid increase in global temperatures coupled with persistent malaria transmission has significant public health ramifications, especially in Sub-Saharan Africa. Indoor thermal comfort and malaria transmission depend on building designs and structural properties of houses. This study assessed the impact of housing modification, combining passive cooling options with vector proofing for indoor temperature reduction and mosquito control.

Methods

Forty houses were randomly allocated to one of four arms: reflective roof, cross ventilation, mat ceiling or control. Additionally, doors, windows and eaves of the intervention houses were screened for vector proofing. Indoor temperatures and humidity were monitored daily whereas, mosquito numbers were assessed monthly. Community knowledge, attitude, and perception were assessed using structured questionnaires.

Findings

Female An. funestus were 67% lower in screened compared to unscreened houses after modification and 6-fold higher before compared to after screening. Houses with reflective roofs and mat ceilings had significantly lower mean day temperature compared to the control, RR=0.08: 95%CI (0.04-0.17) and RR=0.35: 95%CI (0.17-0.74) respectively. At night, houses with cross ventilation and mat ceiling had significantly higher temperatures compared to the control, RR=2.35:95%CI (1.45-3.84) and RR=6.66: 95%CI (4.09-10.85) respectively. Up to 85% of the households expressed willingness to use their resources to modify their houses.

Interpretation

The combinations of cool roofs and vector proofing are effective for the reduction of indoor temperatures and mosquito control in rural African houses. Cross ventilation is prone to user bias as it relies on household members keeping their doors and windows open for air circulation and cooling whereas mat ceilings are effective in keeping indoor temperature low during the day but are relatively warm at night.

Funding

Funded by SeaFright Labs through Habitat for Humanity International.

Health sciences/Risk factors

Biological sciences/Ecology/Climate-change ecology

Climate change associated with extreme weather events is adversely affecting the health of the human population globally [1]. Tropical regions are projected to be most affected due to a faster rise in temperatures [2, 3] and lower interannual variability compared to other areas [4]. The level of impact of the constantly rising temperatures on an individual’s health is dependent on the extent of exposure, indoors or outdoors. While people tend to spend much time indoors, either during occupational engagement or while at home, overheating in the indoor environment due to high temperatures and humidity in the tropics, coupled with poorly designed houses affects health and increases energy demands and carbon emissions [5].

Indoor heat gain in the tropics is largely driven by raised outdoor temperatures, as well as internal heat generated by building occupants, installed equipment and activities such as cooking. To mitigate heat stress resulting from increased environmental heat load, it is important to ensure that the ambient temperature in an indoor living space is maintained within a range, known as a thermal comfort zone (TCZ), where the occupants feel comfortable [6]. Passive cooling and overheating protection in tropical building design is critical in mitigating current and future risks of increasing temperature due to climate change [5]. The inclusion of openings in a building´s exterior surface, e.g. doors, windows and vents [5], is the most common strategy used for indoor cooling during hot seasons in the tropics. These openings facilitate heat transfer between the indoors and outdoors through ventilation i.e., both cross-ventilation and stack-ventilation. The size of these openings relative to the building and their orientation significantly affects the rate of heat transfer between the indoor and outdoor environment. Tragically, in many households, some of these openings are either missing, poorly oriented or disproportionate to the indoor environment to achieve any meaningful cooling indoors.

In addition to providing ventilation into the indoor environment, these openings double up as mosquito entry points [7, 8], further risking the health of the occupants. Mosquitoes that transmit malaria are adapted to enter and feed on humans indoors [9, 10]. A number of studies from western Kenya have shown that human-vector interactions that lead to malaria transmission still occur mostly indoors, throughout the night and morning despite sustained use of long-lasting insecticidal nets (LLINs) [11-15]. The risk of malaria transmission indoors is further exacerbated by the rising temperatures due to climate change crisis leading to lower bed net use during the hot nights [16]. Therefore, house designs that exclude mosquitoes and are comfortable to live in should be a priority in sub-Saharan Africa [17].

House modification, involving blocking of doors, windows and eaves with insect mesh has been demonstrated to reduce malaria transmission [18-21]. Consequently, the World Health Organization has given an interim recommendation for the use of untreated insect screens as a supplementary control measure against malaria [22]. However, blocking these openings could potentially increase indoor temperatures, further jeopardizing the comfort of the occupants. An experimental study in Gambia assessing how house design affects malaria mosquito density, temperature and humidity, observed that blocking of eaves of both thatched and metal-roofed houses increased indoor temperatures, while increasing ventilation in metal-roofed houses made them as cool as thatched houses with open eaves [17]. Critical considerations on house design and its impact on indoor thermal comfort and mosquito control are necessary in every building exercise. To achieve TCZ in the indoor environment while effectively keeping away mosquitoes, this study explored the combined effect of housing modification for passive cooling and vector control in Siaya County, western Kenya.

Study sites

This study was conducted in Kedenge Ratuoro village (0.0242° N, 34.1749° E), near Lake Kanyaboli, Siaya County. The residents are of the Luo ethnic group, subsist on farming, fishing and trade, and live in small houses, clustered into family social units of relatives, called compounds [23]. Malaria transmission in the lake endemic region is stable throughout the year, with a prevalence of 18.9 % in children between 6 months and 14 years of age [16], with Siaya County having a prevalence of 37%. Anopheles gambiae, Anopheles funestus and Anopheles arabiensis are the main malaria vector species. The region has a bi-modal rainfall pattern, with long rains between March and May and short rains between October and December. Annual rainfall ranges from 670 to 2200 mm around Lake Victoria. A temperature suitability index (TSI) for malaria transmission shows that the western Kenya region has ambient temperature and adequate rainfall suitable for endemic malaria transmission [16, 24]. The temperatures are usually between 17°C to 28°C with an average temperature of 24°C in Siaya.

Housing design in western Kenya has been reviewed [23, 25]. Briefly, the houses are typically rectangular, and constructed of stick frames (wattle), compacted soil or cement foundation, and dirt or cement floor. Walls are either mud or cemented while roofs are mostly of corrugated iron sheets. A few houses have clay tile roofs. Doors and windows are unframed or framed with wood to create a jam and sash, but some houses may not have windows. For context, 67.5% (N=40) of the houses in this study did not have windows at the start of the study. The eaves are usually open. The open eaves, in addition to doors and windows, provide ventilation into the houses, allowing entry of light and fresh air. Unfortunately, eaves are also the main route for unlimited entry of mosquitoes into the houses.

Study design. A pilot assessment of the impact of housing modification on indoor mosquito numbers and temperature reduction involving random allocation of selected houses to four different study arms. Forty houses were randomly allocated to either cross ventilation, cool roof, mat ceiling or control, ten houses per arm. Quantitative and qualitative data collections were conducted before and after house modification.

Mobilization and recruitment of study households. Home visits were conducted to enumerate and characterize houses within a section of the study village. A total of 47 compounds with 83 houses where people slept were enumerated. For every active structure in each compound, structural features including wall type, roof type, presence or absence of ceiling, eave type and number of windows and doors were recorded. From the 47 compounds with a pool of 83 houses, 40 houses with mud walls, open eaves, iron roofs and not more than two rooms were selected for the study. A single house was selected per compound.

Structural modification. Structural modification of houses was conducted by a building professional identified based on previous experience with similar modifications. After the randomization of houses into the various study arms, the specific house characteristics including floor area, numbers and sizes of doors and windows, and the presence and sizes of open eaves were collected to guide modification. The building expert established a workshop within the study area where all materials including doors, windows and pieces of timber for eaves and ceiling modifications were prepared before installation in the various houses. Modifications were conducted based on the randomization for the three passive cooling options as follows.

Cross ventilation- Cross ventilation was achieved by installing screened windows on the opposite walls of each room. This involved a complete overhaul of the existing windows and/or the creation of new ones if no windows existed in a house. The windows were made of timber frame with two wooden panels each hanging on two hinges and closing at the centre (Figure 1A and B). A Fibreglass Insect Mesh (FIM) (Streme Limited 12, The Velley Centre, Gordon Road, High Wycombe, Buckinghamshire, United Kingdom, HP13 6EQ) laid between two sheets of Coffee Tray Mesh (CTM) (ALS Limited, Trading Division, Nairobi Kenya) was attached to the window frame for insect screening. The insect screen was installed outwards while the window panels opened inwards. To install the windows, a section of the wall was cut to create space for the window if none existed before or adjusted if the original window was smaller. After the installation of the modified window, the remaining gaps in the wall were filled with mud to achieve the same finish as the original wall.

Cool roof system and insect-proof housing - Iron roofed houses were painted with a reflective white coat to reduce the amount of heat conducted into the house, hence lowering internal temperatures. Two coats of paint were applied on the roofs. Crown Roofmaster^®(Crown Paint Industries, Nairobi, Kenya), an extremely durable weather-resistant, self-priming acrylic resin-based paint with a waterborne topcoat, and matt finish was used (Figure 1C).

False ceiling and insect-proof housing – Locally made papyrus mat ceilings were installed horizontally covering the roof space just above the eaves. Locally sourced round poles were used as bearings and binders for brandering to hold the mat. Wood biddings were nailed below the mats to hold them to the round poles used for brandering (Figure 1D).

Mosquito proofing. All houses that received passive cooling options were screened for mosquito control. The doors were modified by introducing wooden frames and panels in addition to the originally existing door. The existing doors opened inwards while the newly introduced screened doors opened outwards. The screened door panels were made of wooden frames and Fibreglass Insect Mesh laid between two sheets of CTM (Figure 1F). The doors hung on two self-closing hinges to keep them always closed. The windows were also screened as already described above. The eaves were screened by introducing a piece of timber at the edge of the wall just before the eave space and another piece of timber on the roof directly above the wall. Fiberglass Insect Mesh was then attached to the two pieces of timber, hence covering the eave space (Figure 1E).

Temperature and humidity. Daily temperature and humidity were collected every 15 minutes in both modified and control houses from March to July 2023 using Onset HOBO^® UX100-003 data loggers, The data loggers were placed indoors hanging as centrally as possible, but away from walkways and other areas that could potentially interfere with the activities of the household members. Data download was conducted twice during the study period.

Mosquito collection. Mosquito collection was performed indoors using the Centre for Disease Control (CDC) Miniature light trap, model 512. Collections were conducted twice before (baseline) and three times post-modification in each house. The Light traps were set in the sleeping area, next to an occupied bed net, at approximately 1.5m from the floor. The traps were run from 18:00 h to 07:00 h the following morning. During the mosquito collection period, the collector administered a brief questionnaire to collect information on household characteristics, including roof type, wall type, presence of eaves, presence and use of malaria control products such as bed nets, presence of cattle, and number of people that slept in the house the previous night. The location of each house was recorded using the Global Positioning System (GPS). The collected mosquitoes were identified to genus levels as either Anopheles or Culex. The anophelines were further identified morphologically to species level as either An. gambiae s.l. or An. funestus s.l. Data on household characteristics and mosquito information were collected on a CommCare^® (Dimagi Inc., South Africa) application run on an Android tablet and transmitted to a project cloud server.

Collection of social science data. A structured questionnaire assessing community perception, knowledge and attitude towards house modification for vector control and temperature reduction was administered to intervention households before and after house modification. Data was collected on the community’s building practices including reasons for the inclusion or exclusion of certain building elements such as windows, eaves spaces, ceiling, wall and roof types. The questionnaire further assessed the community’s understanding of the relationship between the various building elements and the entry of mosquitoes into houses and indoor heat levels. Additional questions were administered to gauge the community’s perception of changes made to their houses, their potential contribution to house cooling and reduction of mosquito entry as well as their willingness to use their resources to modify their houses. In the post-modification survey, the perceived benefits and risks of house modification and the community’s willingness to continue using the modifications beyond the lifetime of the study was assessed.

Thermal imaging. Thermal images of the houses were taken using the FLIR T450sc^® camera (Teledyne Corporate, 1049 Camino Dos Rios Thousand Oaks, CA 91360, USA). The images were taken early in the morning before sunrise, at mid-day and in the evening after sunset to assess the source of heating in the houses at different times of the day. Images were taken centred on a specific reference point, identified by using a built-in laser pointer, to ensure reproducibility and to compare different times of the day. A set of n= 4 houses was selected, as easily accessible for photographs at all times of the day, and representing the different structural modifications, cool roof, cross ventilation and mat ceiling as well as control.

Data management and analysis

Fielddata were collected using CommCare^® software run on Android tablets. Every participating house was identified by a unique code and a collection code was generated by the tablet for every mosquito sampling effort. These codes were used to track data generated from the different study components for ease of management. Individual mosquitoes from each collection were placed in microcentrifuge tubes labelled with pre-printed barcodes and linked to the field data using a house code and a collection code. Results of species identification by polymerase chain reaction (PCR) were linked to individual mosquitoes by the unique barcode label.

Data analysis was performed using R statistical software version 4.2.1. The risk ratio (RR) was used to assess statistical significance differences in mosquito densities between screened and unscreened houses and indoor temperature levels between houses with passive cooling options and controls. Data were fitted using Generalized Linear Mixed Effects Statistical Models (GLMMs). Since the data were over-dispersed, we used the package Generalized Linear Mixed Models using Template Model Builder (glmmTMB) to fit negative binomial distribution models for the analysis of mosquito numbers. The numbers of female Anopheles and Culex species were assessed as a function of intervention status (screened or unscreened) as fixed effect, while the sampling period was treated as random effect. To obtain the risk ratios (RR) and confidence intervals, we exponentiated the model coefficients. Similarly, glmmTMB was used to fit gaussian distribution models for the analysis of temperature data. Temperature was assessed as a function of passive cooling options (cool roof, cross ventilation, mat ceiling) as fixed effect, while collection period was treated as a random effect.

Mosquito data

A total of 8,297 Anopheles and 2,840 Culex mosquitoes were collected indoors by CDC light traps. More An. funestus, 6,958 (84.2%) were collected before house modification compared to the numbers after modification, 1,303 (15.8%). On the contrary, fewer Culex species were collected before modification, 974 (34.3%) compared to the numbers after modification 1866 (65.7%). Only 2 An. gambiae s.l. were collected in the pre-modification period and 34 in the post-modification period. Therefore, the species was not subjected to further analysis (Table 1).

Relatively high numbers of female and male An. funestus were collected indoors before house modification than after (Figure 2). For female Culex mosquitoes, the numbers were relatively higher in control houses after modification compared to before modification and vice versa in all the modified houses. Male Culex species on the other hand were higher in all houses after modification (Figure 3).

Screening of doors, windows and eaves significantly reduced the number of female An. funestus indoors by 67% in screened houses compared to unscreened ones (RR=0.33: 95%CI 0.13-0.82: P=0.02) after modification. There was a 5.9-fold reduction in the density of female An. funestus indoors after screening (RR=5.91: 95%CI 3.84-9.10: P<0.001). No significant differences were observed in the numbers of male An. funestus between screened and unscreened houses after modification. However, the numbers were significantly higher indoors before screening compared to after screening (RR=13.57: 95%CI 6.87-26.83: P<0.001). Significantly higher numbers of female Culex were observed before compared to after screening (RR=1.69: 95%CI 1.03-2.79: P=0.040), whereas no significant differences in female Culex were observed between screened and unscreened houses post screening. The numbers of male Culex mosquitoes were significantly lower in screened houses compared to unscreened ones (RR=0.37: 95%CI 0.21-0.66: P=0.001) and were significantly higher after screening (RR=0.40: 95%CI 0.27-0.59: P<0.001) (Table 2).

Temperature data

Figure 4 presents mean hourly temperature and humidity distribution over 24 hours in control, cool roof, cross ventilation, and mat ceiling houses before and after modification. In the pre-modification period, the temperature and humidity were relatively similar between the different study arms (Figure 4 A and C). However, after modification, houses with cool roofs had the lowest temperatures and highest humidity levels whereas, control houses had the highest temperature and lowest humidity during the day (Figure 4 B and D). During the day, houses with cool roofs had the lowest temperatures followed by mat ceiling, cross ventilation and control houses were the warmest. Houses with mat ceiling had the smallest mean daily temperature ranges followed by cool roof post modification. Overall, control houses were the warmest during the day with a mean daily temperature of 28.71°C and 54.69% relative humidity, while at night the temperature was relatively lower at 23.36°C and humidity of 71.04%. Cool roof houses were coolest both day and night with a mean daily temperature of 26.02°C and humidity of 64.29% during the day and 23.23°C and 73.33% at night. Houses with cross ventilation were nearly as warm as control houses during the day with a mean daily temperature of 28.43°C and 57.03% humidity and much warmer than control houses at night with mean temperature of 24.19°C and humidity of 68.00%. Finally, houses with mat ceiling were relatively cooler than those of control and cross ventilation during the day with mean temperature of 27.54°C and humidity of 59.17%, however, they were the warmest during the night with mean temperature of 25.42°C and 65.25% humidity (Table 3). Over the data collection period, all the houses were observed to have comparable temperature ranges before modification in March and April. However, as modifications were conducted, houses with cool roofs were observed to have relatively lower temperatures (Figure 5). Based on thermal images, the main sources of heat in the houses were observed to be the roofs and the walls. The roofs were the hottest parts of the houses during the day due to radiation from the sun while the walls emitted much of the stored heat at night, keeping the houses warm after sunset (Supplemental figure 1).

Table 4 presents comparison of mean day and night temperatures before and after modification and between the modified and control houses in the post modification period. Significantly lower mean-day temperature levels were observed in control and cross ventilation houses before compared to after modification RR=0.45: 95%CI 0.39-0.52: P<0.001 and RR=0.45: 95%CI 0.38-0.52: P<0.001 respectively. On the contrary, houses with cool roof and mat ceiling had significantly high temperatures indoor before compared to after modification RR=5.98: 95%CI 5.42-6.60: P<0.001 and RR=1.57: 95%CI 1.42-1.74: P<0.001 respectively.

No significant difference was observed in the mean night temperature in control houses before and after modification. Significantly lower mean night temperatures were observed in houses with cross ventilation and mat ceiling before compared to after modification, RR=39: 95%CI 0.37-.42: P=0.007 and RR=0.07: 95%CI 0.06-0.07: P<0.001 respectively. However, houses with cool roof had significantly higher mean night temperatures before compared to after modification RR=1.70: 95%CI 1.62-1.78: P<0.001.

In the post modification period, houses with cool roof and mat ceiling had significantly lower mean day temperatures compared to control houses RR=0.08: 95%CI 0.04-0.17: P<0.001 and RR=0.35: 95%CI 0.17-0.74: P=0.006 respectively. No significant differences in mean day temperatures were observed between houses with cross ventilation and control. At night, no significant differences in indoor temperature were observed between houses with cool roof and control. However, houses with cross ventilation and mat ceiling had significantly higher mean night temperatures compared to control RR=2.35: 95%CI1.45-3.84: P=0.001 and RR=6.66: 95%CI 4.09-10.85: <0.001.

Cost of modification

Table 5 presents the costs incurred in housing modification for vector proofing (screening) and passive cooling. The exchange rate for US Dollars to Kenya Shillings was $1 = KES 133.66 as of April 2023 when the modifications were conducted. The new windows were screened and oriented to provide cross ventilation hence achieving both vector proofing and cooling indoors at $37.36 per window. Additional screening was conducted on existing windows for vector proofing at KES $8.23 per window. Fabrication and installation of doors for vector proofing attracted a cost of $70.27 per door and eave screening at $69.50 per house. The cost of door, window, and eaves screening for mosquito control per house was estimated at $ 189.12 protecting an average of 4 individuals per house. For passive cooling options, a cool roof attracted the highest cost per house at $145.14 compared to a mat ceiling at $79.01 and cross ventilation at $112.09 for at least 3 windows per house.

Human behaviour data

Knowledge, attitude and perception (KAP) surveys were conducted on 28 and 26 households before and after modification, respectively. The participants interviewed were mostly women, 75% before and 77% after modification. The mean age of the participants was 52 years in the pre-modification period and 54 years post-modification, with the median age being 51 and 58 respectively. The highest level of education attained by the study participants was primary education. The main source of income for the study population was peasant farming 15/28 (54%) and small-scale businesses 7/28 (25%). Other sources of income included skilled labour 2/28 (7%), donations 2/28 (7%) and charcoal burning 2/28 (7%).

The assessment of community knowledge, attitude and perception on housing modification for mosquito control and thermal comfort was targeted at the 30 intervention houses. Of these, 28 and 26 households were interviewed before and after modification respectively. At the beginning of the survey, 12/28 (43%) of the households reported having windows. This reduced to 16/26 (61%) in the post-modification period as some houses received windows for Cross ventilation. The main importance of windows identified by participants was to allow fresh air and light into the house. Other advantages of having windows mentioned by the participants included 1. access to the house when the door is locked, 2. visibility of the outdoors and 3. an outlet for business purposes if part of the house is used as a shop. The main reason given for not having windows installed was inadequate finances with others being; “the current house is a temporary structure before building the desired house”, “I did not think about it then”, “it is a cottage, so it has no window”, “I just built it without windows”, “I left to fix later” and “fear of house collapsing if the windows are installed after building”. The disadvantages of having windows stated during the pre-intervention survey were insecurity, mosquito entry, allowing dust into the house, and being used by witches to harm people in the house. Most participants in both surveys noted that there were no disadvantages to having windows. Of houses that had windows, most of the respondents 66.67% (8/12) and 92.86 (13/14) reported that they opened their windows daily before and after modification respectively.

All the 26 respondents surveyed in the post-intervention period reported that house modification reduced indoor temperatures and mosquito numbers. The majority (77%) of the respondents, perceived a significant reduction, 15% moderate reduction and 4% reported a slight reduction in indoor temperatures.

Most respondents in the pre- (64%) and post- (65%) intervention survey mentioned the importance of building designs that reflect their cultural values especially when building the first house. Fourteen and 20% of respondents in the pre- and post-intervention surveys, respectively, felt the reflection of cultural values in building design is somewhat important while 22% and 15% of respondents in the pre- and post-intervention surveys respectively did not care about the inclusion of cultural values in building. Seventy-one and 96% of the respondents in the pre- and post-intervention periods respectively, expressed willingness to adopt new house designs that are different from their cultural preferences if they help to improve thermal comfort and mosquito control. The proportion of respondents willing to use their family resources increased from 79% in the pre-intervention to 85% in the post intervention survey.

These results demonstrate a significant reduction in mosquito numbers and temperatures indoors in modified houses compared to unmodified ones. The indoor densities of female An. funestus, the main malaria vector in the region, were significantly lower in screened compared to the unscreened houses. Similarly, female Culex mosquitoes were significantly higher in the screened houses compared to unscreened ones. Houses with cool roofs and mat ceilings were significantly cooler during the day while those with cross ventilation showed no significant reduction in indoor temperatures when compared to control houses post modification. At night, the houses with cross ventilation and mat ceiling were significantly hotter compared to control houses while no significant differences were observed between houses with cool roofs and control. In interviews with the participating households, most of the community members expressed willingness to adopt new housing designs and use their resources to improve their houses to control mosquitoes and achieve thermal comfort.

Housemodification for vector control has been demonstrated to be effective in keeping away disease-transmitting mosquitoes. In most of the Western world, mosquito control efforts are mostly targeted outdoors since widespread house screening already excluded mosquitoes from indoor environments [26-28]. Consistent with our findings, several studies in sub-Saharan Africa have demonstrated the effectiveness of screening of eaves, windows and doors for mosquito control [9, 25, 29-33]. Open eaves are the main routes for mosquito entry into houses at night when doors and windows are closed [7, 10], therefore screening of the eaves in addition to doors and windows bears great potential for mosquito control. Housing modification is expected to be particularly effective in regions such as western Kenya where persistently high rates of late-night indoor mosquito biting have been observed despite sustained bed net use [15]. A more recent evaluation in the region observed that while most of the bites experienced indoors were prevented by bed nets, 87% of the remaining bites by An. funestus and88% by An. gambiae for a bed net user still occurred indoors, with mosquito activity peaking early in the morning as people are getting up and out of the protection of their bed nets [12], to much later in the morning in other indoor spaces away from the home environment such as schools [14]. Consequently, house-proofing for vector control offers practical solutions for the protection of the entire housing structures including non-peridomestic structures such as schools against persistent mosquito bites indoors.

Modification of houses for vector control focuses on blocking windows, eaves and doors with an insect mesh material. Such alterations in housing designs potentially alter the indoor environment by reducing the natural flow of air into and out of the building which in turn raises the indoor temperatures. A study in The Gambia demonstrated that screening eaves of thatched and iron-roofed houses made them hotter compared to unscreened ones [17]. This phenomenon is likely to be exacerbated by increasing temperatures because of the global warming crisis. However, our results demonstrate the potential and challenges of housing modification, coupling vector proofing and passive, sustainable cooling options to provide an indoor environment which is not only devoid of nuisance biting and disease-transmitting mosquitoes, but also much cooler than unmodified houses. The dual benefit of house modification in a rural African village may present far-reaching health and economic benefits to the household inhabitants, beyond what has been assessed in the current study, including but not limited to, improved sleep quality and economic productivity, reduced heat stress and reduced malaria incidence, among other health benefits. These should be evaluated in future studies.

Cool roofs were observably the most effective method for reducing indoor temperatures at the hottest hour of the day when compared to control houses. Painting roofs with a white reflective coat reflects much of the sun’s radiations away hence reducing the amount of heat conducted into the house. Based on thermal images, roofs were the main source of heat in the houses during the day and painting them white reduced the indoor mean day temperatures by an average of 2.69°C and up to 11.90°C between the hottest control house and cool roof house. Consistent with these results, a study on experimental houses in Gambia observed houses with white roofs to be consistently cooler and more comfortable than bare metal roofs [34]. Painting the roofs of houses white is a cheap way of making a dwelling more comfortable for the occupants and, because it also leads to reductions in indoor temperatures at night, could potentially increase bed net use in hot humid countries [34]. In addition to the reduction in indoor temperatures, painting the roofs of houses white has the additional benefit of protecting the metal roofs from rusting while improving the beauty of the house.

Mat ceiling was effective at keeping the houses significantly cooler than control houses during the day. However, houses with mat ceiling were observed to be the warmest during the night compared to control houses despite the mats hanging above the eaves. The mat ceiling kept the houses cool during the day by possibly gaining much of the heat from the roof thus providing insulation for the living space indoors. However, at night when the roof cooled off after sunset, the mat ceiling and walls most likely released the heat gained during the day, keeping the house warm. Also, the installation of the mat ceiling horizontally across the roof space reduced the headroom and the space available for air circulation indoors. Moreover, the ceiling locked up a volume of air in the roof space with no vents to release the heated air which possibly heated up the living space. It is however interesting to note, that houses with mat ceilings had the least fluctuation in mean daily temperature of 23.2°C-29.9°C and humidity of 56.5% – 72.7% compared to other cooling options and control houses. These temperature ranges come closest to the human thermal comfort zone with temperatures between 22°C -27°C and a relative humidity of 40%-60%. The indoor thermal conditions of houses with mat ceilings may further be improved by either venting out the heated air from the roof space, aligning the mat ceiling with the roof to increase the space for air circulation within the indoor living area or ensuring cross ventilation within the living space.

Cross ventilation houses did not achieve any meaningful cooling indoors during the day as compared to control houses. Furthermore, the houses were significantly hotter than control houses at night. To be effective, cross ventilation relies on the house members keeping their windows and doors open for airflow and cooling. As observed during the study, the doors and windows were closed whenever the household members were not at home. Besides, not all windows were always open whenever the household members were at home and was thus subjected to user bias. Consequently, cross ventilation was not always achieved as this needed that the windows be opened. Considering that 67.5% of the study houses did not have windows at the beginning of the study, it is highly unlikely that many households would keep their windows open if they existed. Consequently, efforts to achieve cross ventilation would need to be accompanied with sensitization and training of the household members on the benefits of keeping the windows open and the houses well-ventilated. At night, when members of the community go to sleep, doors and windows are always closed. As such, eaves are the only openings available at night for air circulations in cross ventilation houses. The cross-ventilation houses were observed to have significantly higher temperatures at night compared to control houses. This was possibly due to modifications on the eaves for vector proofing as eave screening has been observed to make houses hotter [17]. Consequently, cross ventilations may require combination with other passive cooling options in order to achieve meaningful cooling especially in regions where people keep their windows closed.

The cost of housing modification for vector control was estimated at $ 189.12 protecting approximately 4 individuals per house. Comparatively, 3 long-lasting insecticidal nets (LLINs) cost at $6.5 each for procurement and delivery and would be required to protect each household within the study area for 3 years. While the cost of house screening is relatively high, LLINs provide only partial protection at times when people are asleep and when properly used, additionally, house screening, when done properly would last the lifetime of the structure (>30 years). Studies from western Kenya have demonstrated that the greatest bulk of biting by Anopheles mosquitoes happens indoors, in the morning as people leave the protection of their bed nets [12, 14]. These exposures that occur indoors away from the protection of bed nets are however prevented by house modification as mosquitoes are prevented from entering the houses. We perceive that vector proofing coupled with passive cooling options have far-reaching, sustainable health benefits that remain to be demonstrated.

Limitations: The greatest challenge was experienced in the modification of doors and their use. The introduction of a screened door within the same door space as the existing one was particularly challenging. Often, the existing doors were not standard sizes which meant that every screen door had to be adjusted to fit the available space. The door frames of the original doors were often already destroyed by termites, or worn out due to age, thus needed to be reinforced before installation of the screened doors while the walls of the houses were not usually aligned which made fitting a new door particularly difficult. Additionally, due to cultural predisposition to leave doors open during the day, some households were observed to prop their door screens open which potentially reduced the effectiveness of the screened doors at limiting mosquito entry into the house. A similar observation was made in the Gambia where up to 68% of the doors were propped open [35], raising fundamental questions about the screened doors in rural African housing.

Conclusion and recommendation. House modification coupling vector proofing and passive cooling options have immense potential for controlling persistent indoor malaria transmission while mitigating the impact of the constantly rising temperatures due to global warming in rural African communities. Cool roofs and mat ceilings offer practical solutions for achieving indoor thermal comfort in the low-income communities of Africa during the day. Cross ventilation for passive cooling is prone to user bias as it relies on household members keeping their doors and windows and open for air circulation and cooling whereas mat ceilings are effective in keeping indoor temperature low during the day but are relatively warm at night. Further assessment of the impact of these modifications on health outcomes including sleep quality, heart rate variability, heat stress and strain and malaria transmission is recommended.

KEMRI

Kenya Medical Research Institute

CGHR

Centre for Global Health Research

CDC

Centers for Disease Prevention and Control

TCZ

Thermal Comfort Zone

LLIN

Long Lasting Insecticidal Net

TSI

Temperature Suitability Index

FIM

Fibreglass Insect Mesh

CTM

Coffee Tray Mesh

WBGT

Wet Bulb Globe Temperature

GPS

Global Positioning System

SERU

Scientific Ethics Review Unit

ERC

Ethical Review Committee

Ethics approval and consent to participate.

The study received ethical review and approval from the Kenya Medical Research Institute Scientific Ethics Review Unit (SERU 4796). Before being included in the study, written informed consents were obtained from every household for modification of houses and mosquito collection.

Consent for publication.

Not applicable

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests.

Authors’ Contributions

BA, JS, JO and EO conceive the idea and wrote the protocol. JS and JO secured the funding. BA, TB, JS and EO implemented the study, BA and VM performed the data analysis, JO, DK, MAM provided technical support and BA wrote the manuscript. All authors reviewed and approved the manuscript.

Acknowledgements

The authors would like to thank the study participants and members of Kadenge Ratuoro village for making this work possible.

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Table 1

Numbers of mosquito species collected indoors by CDC light trap from different study arms before and after modification.
	Mosquito species	Control N (%)	Cool roof N (%)	Cross ventilation N (%)	Mat ceiling N (%)	Total
Pre modification	An. funestus Female	725 (33.39)	518 (23.86)	453 (20.87)	475 (21.88)	2171
	An. funestus Male	656 (17.70)	972 (20.30)	619 (12.93)	2540 (53.06)	4787
	An. gambiae Female	0	0	2 (100)	0	2
	An. gambiae Male	0	0	0	0	0
	Culex Female	119 (17.63)	218 (32.30)	164 (24.30)	174 (25.78)	675
	Culex Male	78 (26.09)	95 (31.77)	63 (21.07)	63 (21.07)	299
Post modification	An. funestus Female	402 (51.34)	132 (16.86)	132 (16.86)	117 (14.94)	783
	An. funestus Male	185 (35.58)	57 (10.96)	79 (15.19)	199 (38.27)	520
	An. gambiae Female	14 (45.6)	11 (35.48)	5 (16.13)	1 (3.23)	31
	An. gambiae Male	1 (33.33)	1 (33.33)	0	1 (33.33)	3
	Culex Female	251 (41.97)	141 (23.58)	136 (22.74)	70 (11.71)	598
	Culex Male	606 (47.79)	288 (22.71)	204 (16.09)	170 (13.46)	1268

Table 2

Comparison of mean number of male and female An. funestus and Culex species between different study arms.
Anopheles specie	Level of comparison	Parameters	Mean	RR (95%CI)	p Values
An. funestus female	Post modification	Screened	4.23	0.33(0.13–0.82)	0.02
	Post modification	Not screened	13.40	1
	Pre- and post-modification	Pre-Screening	27.14	5.91(3.84–9.10)	< 0.001
	Pre- and post-modification	Post Screening	6.53	1
An. funestus male	Post modification	Screened	3.72	0.47(0.16–1.37)	0.167
	Post modification	Not screened	3.17	1
	Pre- and post-modification	Pre-Screening	59.83	13.57(6.87–26.83)	< 0.001
	Pre- and post-modification	Post Screening	4.33	1
Culex female	Post modification	Screened	3.86	0.42(0.16–1.09)	0.08
	Post modification	Not screened	8.37	1
	Pre- and post-modification	Pre-Screening	8.44	1.69(1.03–2.79)	0.04
	Pre- and post-modification	Post Screening	4.98	1
Culex male	Post modification	Screened	7.36	0.37(0.21–0.66)	0.001
	Post modification	Not screened	20.20	1
	Pre- and post-modification	Pre-Screening	3.74	0.40(0.27–0.59)	< 0.001
	Pre- and post-modification	Post Screening	10.57	1

Table 3

Measure of maximum, minimum and mean daily temperatures and humidity in control, cool roof, cross ventilation, and mat ceiling house post modification.
Category	Time of the Day	Minimum Temperature (°C)	Maximum Temperature (°C)	Mean (°C)	Mean RH (%)
Control	Day	18.06	46.98	28.71	54.69
Control	Night	18.27	42.55	23.36	71.04
Cool Roof	Day	18.78	35.08	26.02	64.29
Cool Roof	Night	18.85	30.15	23.23	73.33
Cross ventilation	Day	20.06	39.7	28.43	57.03
Cross ventilation	Night	19.61	30.28	24.19	68.00
Mat Ceiling	Day	20.20	36.40	27.54	59.17
Mat Ceiling	Night	20.96	35.99	25.42	65.25

Table 4

Comparisons of mean day and night temperatures in houses in the different study arms before and after modification and between houses with the passive cooling options and control houses post modification.
Level of Comparison	Period	Intervention	Mean Temp (°C)	RR (95%CI)	P Values
Pre- and post-modification	Day	Control Pre-modification	27.85	0.45(0.39–0.52)	< 0.001
		Control Post Modification	28.71	1
		Cool roof Pre-modification	27.74	5.98(5.42–6.60)	< 0.001
		Cool roof Post modification	26.02	1
		Cross ventilation Pre-modification	27.7	0.45(0.38–0.52)	< 0.001
		Cross ventilation Post modification	28.43	1
		Mat ceiling Pre-modification	28.31	1.57(1.42–1.74)	< 0.001
		Mat ceiling Post modification	27.54	1
	Night	Control Pre-modification	23.36	0.99(0.93–1.05)	0.74
		Control Post Modification	23.29	1
		Cool Roof Pre-modification	23.82	1.70(1.62–1.78)	< 0.001
		Cool roof Post modification	23.23	1
		Cross ventilation Pre-modification	23.27	0.39(0.37–0.42)	0.007
		Cross ventilation Post modification	24.19	1
		Mat ceiling Pre-modification	22.62	0.07(0.06–0.07)	< 0.001
		Mat ceiling Post modification	25.42	1
Post-modification	Day	Cool roof	26.02	0.08(0.04–0.17)	< 0.001
		Cross ventilation	28.43	0.82(0.39–1.72)	0.59
		Mat ceiling	27.54	0.35(0.17–0.74)	0.006
		Control	28.71	1
	Night	Cool roof	23.23	0.91(0.55–1.48)	0.703
		Cross ventilation	24.19	2.35(1.45–3.84)	0.001
		Mat ceiling	25.42	6.66 (4.09–10.85)	< 0.001
		Control	23.36	1

Table 5

Costs of housing modifications
Category	Item	Cost per unit (KES)	Number of units	Total cost (KES)	Total cost ($)
Screening	New windows (Cross ventilation)	4,994	33	164,802	1,232.99
	Doors	9,392	32	300,544	2,248.57
	Screening of existing windows	1,100	13	14,300	106.99
	Eave screening	9,290	30	278,700	2,085.14
	Subtotal			758,346	5,673.69
Passive cooling	Cool roof	19,400	10	194,000	1,451.44
	Mat ceiling	10,560	10	105,600	790.06
	Subtotal			299,600	2,241.51
Total				1,057,946	7,915.20
Cost of screening per house				25,278.20	189.12

There is NO Competing Interest.

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Passive cooling options in combination with vector proofing for indoor heat reduction and mosquito control: a pilot study in rural western Kenya

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