Main findings
A number of self-reported instruments have been developed in epidemiological studies across a range of countries to assess domestic environmental exposures in children and adults with asthma. These capture data on a variety of factors such as indoor exposure to damp, biological irritants, and environmental tobacco smoke, which are associated with adverse asthma outcomes. These instruments typically included a combination of multiple choice and binary response questions, some of which were adapted from large epidemiological studies, with recall periods mostly within the previous 12 months. Many studies reported relatively high response rates exceeding 50%.
However, our review also found considerable variation in these self-reported instruments, with little uniformity among the included questions. Most instruments also lacked adequate coverage of key domestic environmental exposures relevant to respiratory health. Critically, most studies did not describe the methods for instrument development or validation, highlighting a need for transparently reported and validated instruments to be developed.
Relationship to other studies
According to our review, the most frequently assessed self-reported domestic environmental exposure was exposure to damp and 79% of the included studies reported a significant association with adverse asthma outcomes. This is in line with strong evidence on the association between objectively measured indoor dampness and adverse respiratory outcomes.(13),(14) However, there was no uniformity in the method of evaluating self-reported dampness. Some studies had used scoring systems to evaluate self-reported dampness (15),(16) while others had assessed this using binary response questions.(17),(18)
There is also evidence of a causal relationship between exposure to biological irritants like mould and adverse asthma outcomes.(8) In our review, exposure to biological irritants was evaluated by 71% of the studies and 29% reported that it was associated with adverse asthma outcomes. However, two studies reported exposure to pets had a protective effect on asthma outcomes, which aligns with previous research suggesting higher levels of indoor pet or pest allergens in early life are associated with a lower risk of asthma.(19) This suggests that biological exposures need to be carefully categorised into those that may be protective (e.g. exposure to pets in early life) and those that may be harmful (e.g. exposure to mould) to asthma outcomes.
The association between household environmental tobacco smoke and adverse asthma outcomes is well established.(20) In our review, environmental tobacco smoke was assessed in 69% of studies but only 24% reported an association with adverse asthma outcomes. This suggests that the self-reported measures of indoor second-hand smoke identified in our review may not adequately capture this exposure and other validated self-reported measures are needed.
Previous literature has shown indoor ventilation to be a significant factor in determining asthma outcomes and good ventilation has been identified as an important measure for improving the indoor environment for people with asthma. (8) Poor ventilation increases the exposure to indoor respiratory irritants by increasing their concentration and the length of exposure time within the household. Previous studies also reported that poor domestic ventilation is associated with increased risk of asthma exacerbations in children.(21) However domestic ventilation was not included in 80% of the self-reported instruments included in our review.
Only 34% of the included studies had assessed the use of indoor air quality appliances such as air purifiers in relation to asthma outcomes and found mixed results in their association. There is growing evidence that poor indoor air quality is a major contributor to respiratory morbidity and indoor air quality appliances have the potential to improve these health outcomes. (22) However, further studies are required to elucidate the impact of these indoor air quality improvement measures on asthma outcomes.
Indoor volatile compounds, such as chemicals from cleaning products, are emerging as a new class of domestic environmental risk factor for adverse respiratory effects, particularly in high income countries. (23), (24) In our review, 34% of the included studies had evaluated these domestic exposures in relation to asthma outcomes and 14% reported a significant association between indoor volatile compounds and asthma outcomes.
Even though adverse health effects due to combustion of solid fuel for household cooking and heating are commonly reported in studies of rural households in developing countries, (24) 59% of studies in our review, which were based in middle- and high-income countries, also evaluated the impact of indoor combustion byproducts on asthma outcomes. However, only 14% of studies in our review demonstrated a significant association between indoor combustion byproducts and asthma outcomes.
Indoor temperature had been evaluated by only one study in the present review, even though it is likely to be associated with asthma-related morbidity.(25),(22) Results from previous studies have shown that extreme hot and cold temperatures both increase the incidence of asthma attacks, albeit through different mechanisms.(26) Therefore, assessment of indoor temperature by self-reported measures may still be important in evaluating domestic environmental exposures relevant to asthma outcomes, but further research is needed to investigate how best to capture these data.
Adverse domestic environmental exposures and poverty are interrelated, and poverty is often a confounding factor in most health and environmental studies. (8) However, only a few studies included in our review had captured data on poverty and its impact on domestic environmental conditions (27) and asthma outcomes. (28) (29) Future research should ensure that measures of poverty are captured and potentially incorporated into measures of domestic environmental exposures to ensure this important risk factor is adequately accounted for.
A lack of information on the reliability and validity of most self-reported instruments included in this review suggests a need for detailed psychometric testing of self-reported instruments. Engman et al (2007) validated a self-reported instrument assessing domestic environmental exposures to dampness, ventilation and household type and quality, by comparing self-reported responses with external inspections. (30) Cohen’s kappa was used to measure the agreement between inspectors’ observations and self-reports. The self-reported instrument was found to be reliable in evaluating technical parameters like the type of house and ventilation system, particularly for single family houses. It was less reliable at evaluating indications of dampness and mould. However, the time lapse of 18–24 months between administration of the self-reported instrument and the inspections could have influenced the findings. Suggested ways of improving the validity of such instruments were the inclusion of simple drawings, pictures or examples to visually demonstrate the exposures being assessed.(30)
Strengths and limitations
We included a large number of studies that used self-reported instruments to assess domestic environmental exposures in people with asthma, published over the last two decades, and provided insights on their development, content and association with asthma outcomes.
However, our review has a number of limitations. We only included studies published in English and did not explore studies published in other languages. We also only included studies from high- and middle-income countries and 53% were from China, Sweden, the USA, and Australia. Our findings are therefore not generalisable to low-income countries. Asthma is a heterogenous disease, and the impact of the indoor domestic environment is influenced by age, geography, social, cultural and economic factors. It is therefore challenging to implement self-reported measures universally, highlighting the need to have locally adapted instruments that are contextually appropriate for the populations under investigation.
We acknowledge that our content assessment was quantitative, and further work is needed to provide a more detailed qualitative assessment of existing instruments.
82% of the studies evaluating domestic environmental exposures were cross sectional studies. Thus, the reported associations between self-reported domestic environmental exposures and asthma outcomes need to be interpreted with caution and should be validated against longitudinal studies.
We observed a lack of consensus in definitions of domestic environmental exposures assessed by self-reported instruments which made comparisons between instruments challenging. Furthermore, many of the instruments included in our review used multiple choice and binary response questions to evaluate domestic environmental exposures which may have led to measurement or misclassification bias. A few self-reported instruments had attempted to minimise this by incorporating scales or scoring systems to assess exposures like dampness, (15), (16) ventilation and indoor cleaning (31). Appropriately developed scales and weighted scoring systems may improve the measurement properties of such instruments and should be considered in further studies.
Implications for practice, policy and research
The findings of our review have implications for future research evaluating the prevalence and relationship between domestic environmental exposures and asthma outcomes. There is a need for consensus on the definitions of domestic environmental exposures affecting asthma outcomes and the methods for capturing these data for clinical practice and research using self-reported instruments. This could enable researchers and healthcare providers to develop a more robust understanding of the burden of adverse domestic environmental exposures, and their relationship to poor asthma outcomes.
Our review highlights the need for a standardised and validated self-reported instrument that adequately covers the breadth of relevant domestic environmental exposures that affect asthma outcomes. Greater clarity and consistency are also required in the method of administration, scoring and interpretation of the findings of such instruments. Use of a validated self-reported questionnaire in routine clinical practice to assess domestic environmental exposures in asthma patients could guide targeted management and social interventions.(32) This could in turn enable policymakers to invest in evidence-based interventions for the benefit of asthma patients.
62% of self-reported instruments in our review had used a recall period of 12 months or less in assessing domestic environmental exposures. However, 16% of the included instruments had a recall period of more than 12 months which increases the risk of recall bias. Future development of self-reported instruments for assessing domestic environmental exposures should carefully consider the impact of recall bias when determining recall periods. 73% of the instruments had a response rate of greater than 50%, indicating the potential feasibility of implementing similar instruments successfully in routine asthma care.
Two thirds of the selected studies were conducted among children and childhood asthma was the outcome of interest for 54% of the included studies. (11) Children, due to their size, are at closer proximity to the ground than adults and spend considerable time in the domestic environment, resulting in a higher risk of exposure to indoor pollutants.(34) Further research is needed to understand whether separate self-reported instruments are required for children and adults to accurately assess relevant domestic environmental exposures.