2.1 Study site
Hotter and more humid air masses influence subtropical and tropical climate areas more than other climates [24]. In these areas, indoor environments without an adequate cooling system can cause indoor overheating [25]. In Florida, statewide average temperatures are higher than 30 degrees Celsius during the summer, and some residents lack AC units to manage cool indoor environments. According to the AHS, in 1980, 15.66 % of Florida households had no AC, 54.79% of households had central AC, 17% of one individual room unit, and 10.27% of houses had two or more individual room units. Therefore, this study investigated residential AC availability at the census tract level in Florida from the historical records up through 2019 using a novel real estate dataset to reflect more current/updated estimates of AC saturation.
We further assessed the patterns of AC ownership within Duval County (Jacksonville). Duval is rapidly growing and contains metropolitan (Jacksonville) as well as urban, suburban and rural areas (Figure 1). During the Florida land boom in the 1920s, Jacksonville was considered a "Gateway to Florida" [26]. Jacksonville’s population tripled between 1950 to 2000 [27], and the city hosts 949,611 residents as of 2020, making it the 13th most populous city in the US. and the fourth largest metropolitan city in Florida. Jacksonville has the continental US's largest city area (747 mi2).
According to American Community Survey (ACS) 2019, the racial composition of Jacksonville was White 58.22%, Black or African American 30.95%, Asian 4.76%, two or more races 3.64%.
To increase understanding of Duval County’s general socioeconomic characteristics, our analysis was conducted within Duval County Health Department Health Zones (HZ). In 2015, HZ grouped zip codes with similar economic, educational, geographic, political, and social boundaries (Yu, 2016). For instance, according to ACS estimates census tract 2014, HZ1 is the most populated urban area, and more than 80% of the residents are African American. HZ5 is considered to be the most rural area, while HZ3 and HZ6 contain less than 20% African Americans. Hispanic residents are primarily located in HZ2 (41%), HZ4 (23%), HZ3 (19%) in Duval County. HZ1 showed large health disparities in many indicators such as low income, low education, and high unemployment rates compared with the other HZs [28].
2.2 Data
2.2.1 Property data
‘Estated,’ which has access to more than 150 million properties nationwide, provided housing information. The dataset includes detailed information on individual properties, including building structure, market value, taxes and assessment, and historical deeds. Estated provided information on 72,954,014 residential properties in Florida. We excluded the properties that were missing either the year of construction or AC types which left 42,679,440 properties for the analysis. Lafayette County, Liberty County, Miami-Dade County, Polk County, and Leon County were the top 5 counties that had the most missing data of AC ownership (Appendix).
2.2.2 Contemporary and historical socioeconomic variables
2.2.2.1 Contemporary socioeconomic variables
Intuitively, AC ownership is related to residents’ socioeconomic position. Biddle (2008) investigated the growth of AC prevalence with historical census data and showed that AC ownership doubled up in the 1980s compared to the 1960s due to the development of affordable AC units, declining cost of electricity, increasing household incomes, and new housing developments. Several studies found that residential building characteristics were related to AC ownership [19, 23, 29, 30] such as year built showed a positive relationship with AC prevalence and [21] the value of the property, and total square ft of property showed a negative relationship with AC ownerships [31]. We referred to the study mentioned above and collected ten variables from 5 year ACS (2019) at the census tract level. In Florida, this sample includes approximately 3.54 million housing units (Table 1).
2.2.2.2 Historic sociodemographic variables
Recent studies argue that redlining, or historic disinvestment in neighborhoods with historically high proportions of foreign-born populations, are related to contemporary health inequalities [32, 33]. Historical development and neighborhood characteristics may also influence building amenities such as AC availability [34]. Since historic redlining information is only available for three cities in Florida, we included census tract-level Black/African American population from the US 1970 decennial census [35]. This date follows the passage of the 1968 Fair Housing Act and likely contains vestiges of past discriminatory practices.
2.2.3 Urbanicity variable
The United States Department of Agriculture (USDA) (2010) classifies census tracts into 11 urban and rural subtypes based on population density, urbanization, and commuting patterns. The study collapsed the ten original urban/rural codes into four classes: Metro Urbanized Areas (UAs) (Metropolitan area core UA, Metropolitan area high commuting UA, Metropolitan area low commuting UA), Metro Urban Clusters (UCs (Micropolitan area core large UC, Micropolitan high commuting large UC, Micropolitan low commuting large UC), Town UC (Small-town core small UC, Small-town high commuting small UC, Small-town low commuting small UC), and Rural (outside a UA or UC) (Table 1).
Table 1
Variable categories and data sources
Category |
Variables |
Sub-Categories |
Data Source |
Property |
Built year |
- |
Estated (2021) |
|
Air conditioning types |
None, Central, Other |
Estated (2021) |
|
Value |
- |
United States Census Bureau (2019) |
|
The average number of rooms per residence |
|
United States Census Bureau (2019) |
Contemporary sociodemographic |
% 65 and over, lives alone |
- |
United States Census Bureau (2019) |
|
% Households with complete plumbing facilities |
|
United States Census Bureau (2019) |
|
% Renter occupied |
- |
United States Census Bureau (2019) |
|
% Black or African American |
- |
United States Census Bureau (2019) |
|
% Hispanic or Latino |
- |
United States Census Bureau (2019) |
|
Median household income |
- |
United States Census Bureau (2019) |
Historic demographic |
% Black or African American |
- |
IPUMS NHGIS, (1970) |
Urbanicity |
Urban and rural codes |
1. Metro_UA 2.Metro_UC 3.Town_UC 99: Not coded |
United States Department of Agriculture (USDA) (2013) |
2.2.4 Data preprocessing
We extracted built year, total property square footage, and AC types from the Estated dataset and aggregated the AC dataset to the census tract level. The Estated dataset provided individual properties’ AC type information such as central, chilled water, evaporative cooler, geothermal, packaged AC unit, partial, refrigeration, ventilation, wall unit, window unit, yes, none, and others. Some studies differentiated cooling effectiveness among various AC types (Quinn, Kinney, & Shaman, 2017; Waugh et al., 2021). Waugh et al. (2021) found that houses with room AC units had an average of 2 degrees Celsius higher indoor temperature than houses with central AC. Quinn et al. (2017) suggested portable AC was closer to not having an AC than central AC based on room temperature.
Some jurisdictions only reported the presence or absence of AC which constrained the statewide analysis of residential AC types. Thus, the study grouped AC into three categories: any AC (packaged AC unit, chilled water, geothermal, commercial unit, central AC, wall unit, window unit, evaporative cooler, and other AC unit), no AC (none and ventilation), and missing data. The study did presume that areas with 100% of any AC automatically had no households without AC since there were residences with missing AC information across the study area. Next, we calculated the percentage of each AC type at the census tract level by dividing it by the total number of units reporting AC availability.
We calculated the % of renter-occupied and % households with complete plumbing facilities divided by the number of total housing units. After that, we merged the Estated data with socioeconomic variables data we gathered from ACS with census tract based on the Federal Information Processing Standards code. The contemporary socioeconomic variables were converted to proportion of the population (Table 1). We calculated % Black or African American from the historic total population Black or African American and matched it with the contemporary census tract.
To compare the effect of each variable on the dependent variable, we standardized all variables by converting them to Z-scores (subtracting the mean and dividing by the standard deviation). We standardized to avoid over-emphasizing one variable’s effect on the dependent variable.
2.3 Statistical analysis
This analysis aimed to identify the spatial autocorrelation of AC availability in Florida. We separately applied Moran’s I test to the percentage of anyAC or noAC with the ‘spdep’ package in R [39] to examine spatial autocorrelation in Florida. Moran's I statistic is a global spatial autocorrelation statistic designed to test the null hypothesis of complete spatial randomness. To conduct the Moran’s I statistic test, we defined the neighborhood of census tract polygons with the queen criterion of contiguity. We applied a binary weights matrix without row standardization, which gives more weight to areas with more neighbors.
Among the total of 4,152 census tracts from 67 counties, some census tracts contained missing values. Therefore, we included 1,172 census tracts from 26 counties for any AC and 1,166 census tracts from 23 counties for the no AC analysis. Based on the available data, the Duval County analysis examined a different number of any AC census tracts (173) versus the no AC analysis (134). Due to missing data, the study did not presume neighborhoods with 100% AC saturation imply no households with AC does not infer that only having any AC records means 0% no AC
We applied Local Indicators of Spatial Association (LISA) to AC availability in Florida with the ‘spdep’ package in R version 4.1.1 [39] at the census tract level. We operationalized neighborhoods using queen contiguity without row standardization to conduct LISA analysis. The sub-analysis reports specific information for Duval County.
Next, we fit a regression model to analyze the relationship between AC availability and socioeconomic variables in Florida. To consider potential multicollinearity between independent variables, we retained explanatory variables when the Variance Inflation (VIF) was less than 5. The percentage of households without AC was the dependent variable, and the rest were independent variables (Table 1).
Spatial regression models, including spatial error (SEM) and spatial lag model (SLM), are commonly applied to handle residual autocorrelation [5]. The spatial lag model (SLM) can be used for a diffusion process that looks at how one event increases the likelihood of similar events in neighborhood areas [40]. SEM treats spatial dependence as a nuisance, which means SEM removes the autocorrelation by adding a spatial correlation term to the residuals [41]. However, some studies suggested that applying either SEM or SLM can lead to erroneous conclusions[42, 43]. For example, the SLM model cannot account for spatial correlation in the error term, and SEM cannot provide information about indirect effects of neighbors [43]. To capture both error dependence and spatially lagged dependence, we applied the Spatial Durbin Model (SDM) [43, 44]. The SDM is considered more robust since it considers both local and global spatial effects with no prior restrictions on the magnitude of potential direct and indirect effects [42].
SDM provides direct effect and indirect independent variable effect estimates. The direct effect refers to the changes of dependent variable effects on a census tract, and this also considers how the census tract changes affect its neighbor census tract [45]. Indirect effect or spatial spillover effect refers to the neighboring dependent variable’s impact on their census tracts’ dependent variable, while the total effect sums the direct and indirect effects [45]. The results report the standardized beta coefficients (one standard deviation change), 95% confidence intervals, and p-values.