2.3.1 Heat Health Risk Assessment
Heat health risk is described as a function of heat hazard, exposure, and vulnerability. It was calculated by multiplying three indices: heat hazard, exposure, and vulnerability (Fig.
2). The three indices are regarded as having equal weights, and the specific calculation method is as follows:
$${\text{HHRI}} = {\text{HHI}} \times {\text{EI}} \times {\text{VI}}$$
(1)
where HHI represents the heat hazard index, EI represents the exposure index, VI represents the vulnerability index, and HHRI represents the heat health risk index.
To map the spatial distribution of daytime high-temperature hours (DHs) and nighttime high-temperature hours (NHs), population density, vulnerability, and heat health risk, the standard deviation classification method was used. This method classifies the data based on the mean and standard deviation; the greater the difference is between the data values and the mean, the more levels the data set is divided into.
(1) Heat Hazards
In previous studies for Beijing by Du et al. (
2014) and Zhang and Li (
2020), only 18 shared meteorological stations or interpolation data based on limited meteorological stations were used when assessing heat hazards. In our study, the data from 225 meteorological stations in Beijing for 2011–2020 were obtained by cooperation with the Beijing Meteorological Disaster Prevention Center. This provided an expanded dataset with the largest number of stations and abundant data. The hourly temperature data during the daytime (7:00–18:00) and nighttime (18:00–7:00) in the 10-year period were sorted from low to high, and the 90% quantile was taken as the high-temperature threshold. The daytime high-temperature threshold was 33.1 °C, and the nighttime high-temperature threshold was 27.9 °C. The cumulative number of daytime hours of each site in which the temperature exceeded the high-temperature threshold in summer (June to August) in the 10-year period was calculated and abbreviated as DHs. The cumulative number of hours of each site in which the temperature was above the high-temperature threshold during summer nights over the decade was calculated and abbreviated as NHs. The DHs and NHs quantified the extreme heat hazards in Beijing by combining the intensity and duration of high temperatures.
Land cover (cropland, woodland, grassland, shrubland, wetland, water, urban land, and bare land), population density, DEM, and GDP were used as the predictors of the spatial distribution of high temperature. Then, high-temperature interpolation models of DHs and NHs were established based on urban land surface features. The high-temperature interpolation model is as follows:
$${\text{DHs}}\& {\text{NHs}} = f(dem,land_{n} ,pop,{\text{GDP}})$$
(2)
Buffer analysis is based on point, line, and surface features, and it creates a polygon layer of the buffer zone within a certain width around the feature of concern. We overlay this layer and the target layer to analyze and obtain the required results. To identify the spatial scale at which the strongest correlation between various factors and DHs and NHs is observed, 100 buffer widths ranging from 1 to 100 km were set around the 225 meteorological stations, and the land cover grid data were cropped with the vector files of the buffers. Using the spatial analysis function of ArcGIS, the areas of eight land cover types in the 100 buffer zones at each of the 225 meteorological stations were obtained (8 × 100 = 800 variables). The DEM, GDP, and population density data were point data (3 variables). As a result, 803 predictor variables were prepared.
Pearson correlation coefficient can be used to calculate the type of linear relationship between two variables (positive, negative, none) and the strength of this relationship (weak, moderate, strong). We used Pearson correlation coefficient to analyze the correlation between each predictor variable and DHs and NHs. Due to the low correlations between shrubland, wetland, water areas, and bare land and DHs and NHs within the 1–100 km buffer, these variables were removed. Then, the buffer widths with the highest correlations with DHs and NHs were determined among the 100 buffer widths for the remaining land cover types—cropland, woodland, grassland, and urban land. Stepwise linear regressions were performed with DHs and NHs as dependent variables, and cropland, woodland, grassland, urban land, population density, DEM, and GDP as independent variables. Then, multiple linear regression equations, called high-temperature interpolation models of DHs and NHs (HIM-D, HIM-N) for the study area, were obtained. The equation for DHs is as follows:
$${\text{DHs}} = 1187.342 - 1.170 \times dem + 0.1 \times urbanland + 0.018 \times pop - 0.111 \times grassland$$
(3)
and the equation for NHs is as follows:
$${\text{NHs}} = 665.715 - 0.654 \times dem + 30.505 \times urbanland + 0.025 \times pop - 0.06 \times grassland$$
(4)
The statistical analysis by F test showed that the correlations were highly significant at
p < 0.01; in other words, the linear relationships between the predictor variables entering Eqs. (
3) and (
4) and the cumulative hours of high temperature were very close. From the adjusted
R2 (HIM-D: 0.76; HIM-N: 0.84), the dependent variables have a high degree of explanation for the independent variables and the models fit well.
Regular 1 km × 1 km grids and grid points were generated in the study area, the values of the independent variables and the values of the dependent variables, DH and NH, of each grid point were calculated, and then these values of the grid point were assigned to the grid to obtain the spatial distribution map of the high-temperature hazard with a resolution of 1 km.
(2) Exposure to Hazard
Exposure refers to the exposure of lives and property, human health, ecosystems, resources and the environment, and infrastructure to certain hazards (Yin et al.
2013). As the vulnerability indicators selected in this study mainly involved the population, exposure in this study was referred to as the population exposure to heat hazards. In the past, scholars have used the sixth population census data to calculate the population exposure at the county level. In this study, the seventh population census data were used to calculate the population density of each subdistrict by dividing the permanent population of each subdistrict by the area of each subdistrict, and the unit was persons/km
2.
(3) Vulnerability Assessment
Different from previous vulnerability studies based on the county scale, this study calculated vulnerability at the subdistrict scale in Beijing. Factors unfavorable to people’s coping with heat hazards were selected as vulnerability indicators: (1) Proportion of resident female population. Compared to that of men, the physical resistance of women may be weaker, and their economic strength is generally inferior to that of men; therefore, it is more difficult for them to recover after disasters (Cutter et al.
2003; Xie et al.
2015b); (2) Proportion of the resident population aged 0–4. Compared to young adults, children have poorer physical resistance, less self-protection ability, and poorer disaster response capabilities (Cutter and Finch
2008); (3) Proportion of permanent population over the age of 65. Elderly individuals over the age of 65, especially those suffering from cardiovascular and cerebrovascular diseases, are more susceptible to high temperatures due to body dysregulation. They are also susceptible to high-temperature diseases (Cutter and Finch
2008); (4) Proportion of resident minority population. Cultural and language barriers hinder ethnic minority evacuation and rescue during disasters (Reid et al.
2009); (5) Proportion of people living alone. People who live alone have less contact with their surroundings and may not receive effective assistance when disasters occur (Reid et al.
2009); and (6) Proportion of the population with an education level below high school. It is generally believed that people with higher education levels have better disaster awareness. Moreover, they have stronger economic strength for disaster adaptation and post-disaster recovery (Uejio et al.
2011). The entropy weight method was adopted to determine the weight of each indicator (Table
1), and then the addition method was adopted to generate the vulnerability index:
$${\text{VI}} = 0.0251 \times I_{1} + 0.0554 \times I_{2} + 0.1293 \times I_{3} + 0.4321 \times I_{4} + 0.2777 \times I_{5} + 0.0804 \times I_{6}$$
(5)
where VI represents the vulnerability index;
I1,
I2,
I3,
I4,
I5,
I6 represent indicator 1 (proportion of resident female population), indicator 2 (proportion of resident population aged 0–4), indicator 3 (proportion of permanent population over the age of 65), indicator 4 (proportion of resident minority population), indicator 5 (proportion of people living alone), and indicator 6 (proportion of population with education level below high school), respectively.
Table 1
Weights of six vulnerability indicators and four adaptability indicators
Vulnerability | Indicator 1 | 0.0251 |
Indicator 2 | 0.0554 |
Indicator 3 | 0.1293 |
Indicator 4 | 0.4321 |
Indicator 5 | 0.2777 |
Indicator 6 | 0.0804 |
Adaptability | Indicator 7 | 0.0302 |
Indicator 8 | 0.5038 |
Indicator 9 | 0.2214 |
Indicator 10 | 0.2446 |
2.3.2 Adaptability Assessment
In this study, the factors that were beneficial to people’s mitigation of heat hazards were selected as the adaptability indicators: (1) Air conditioner ownership per 100 households. Indoor cooling equipment, such as air conditioners, can effectively alleviate high temperatures and prevent high-temperature injuries to a certain extent (Klinenberg
2003); (2) GDP. To some extent, GDP can reflect people’s living and economic standards. Areas with a high quality of life and high economic level usually have more heat reduction equipment, medical institutions, and infrastructure to alleviate high temperatures; (3) Proportion of medical institutions within a 10-min walking distance. The quantity and distribution of medical resources directly affect whether timely treatment can be obtained after a disaster (Cutter et al.
2003); and (4) Proportion of parks within a 10-min walking distance. Parks are effective places in which to find both relief and escape from high temperatures, and the number and layout of parks reflect the comfort of people’s lives (Johnson et al.
2012). We set the 10-min walking distance to 600 m. The entropy weight method was adopted to determine the weight of each indicator (Table
1), and addition was adopted to generate the adaptability index:
$${\text{AI}} = 0.0302 \times I_{7} + 0.5038 \times I_{8} + 0.2214 \times I_{9} + 0.2446 \times I_{10}$$
(6)
where AI represents the adaptability index;
I7,
I8,
I9,
I10 represent indicator 7 (air conditioning ownership per 100 households), indicator 8 (GDP), indicator 9 (proportion of 10-min walking range of medical institutions), and indicator 10 (proportion of 10-min walking range of park green space), respectively.