In a recent study published in the journal Science of the Total Environment, researchers from China examined the impact of interactions between environmental factors such as heatwaves, air pollution, and the distribution of green and blue spaces on hypertension.
Extreme heat exposure affects cardiovascular health, and studies have found that changes in blood pressure (BP) are significantly correlated to ambient temperature. With the steady increase in global average temperature due to climate change, extreme weather events such as heat waves are expected to have a serious impact on human health, especially the cardiovascular health of the elderly population.
Furthermore, the increase in air pollution, especially involving particulate matter smaller than 2.5 µm in diameter (PM2.5), has also been associated with hypertension. Elevated environmental temperature can also increase the PM2.5 concentrations in the atmosphere, having a cumulative detrimental effect on health. However, studies have shown that the expansion of green and blue spaces comprising areas with vegetation and waterbodies, respectively, could have beneficial effects on health by counteracting the action of increasing heat and air pollution. Blue and green spaces are known to provide a cooling effect by increasing atmospheric humidity and counteracting the effect of daytime near-surface, shortwave radiation.
About the study
In the present study, the researchers analyzed data from the Chinese Longitudinal Healthy Longevity Survey (CLHLS), which assessed the relevant factors affecting the health of adults above 65 years of age. This survey began in 1998, with intermittent follow-ups till 2018. For the present prospective cohort study, the researchers used the CLHLS data from the years 2008, 2011, 2014, and 2018. Participants who had a baseline diagnosis of hypertension, or who did not have complete data on demographic factors, blood pressure, and other key variables were excluded from the study.
Hypertension assessments were based on BP measurements taken during follow-ups using a mercurial sphygmomanometer. Two readings of the mean systolic and diastolic blood pressures each were recorded, and hypertension was defined as systolic BP equal to or greater than 140 mm Hg and diastolic BP equal to or greater than 90 mm Hg. Self-reported hospital diagnoses of hypertension were also considered.
Air pollution was inferred from data on ground-level concentrations of PM2.5 collected using satellite, monitor, and simulation-based methods. Aerosol optical depth retrievals from various satellite products were combined to obtain PM2.5 concentration data at a 1.1 km × 1.1 km resolution for the duration of the study. Based on previous studies, the air pollution (PM2.5) exposure window was set at one year.
Exposure to green spaces was determined from normalized difference vegetation index data obtained at a spatial resolution of 500 m from the Moderate-resolution Imaging Spectroradiometer sensor (MODIS) database. Plant Functional Type (PFT) data on Shannon’s diversity index and land cover were used to determine the open water body areas, which were proxies for blue spaces.
Daily maximum temperatures recorded between 2007 and 2018 from over 2400 meteorological monitoring stations in China were used to assess heatwaves. Nine heatwave definitions based on combinations of temperatures equivalent to the 92.5th, 95th, and 97.5th percentiles of the daily maximum temperatures and three durations greater than or equal to two, three, or four days were used in the analysis.
Information on other covariates included gender, age, area and geographic region of residence, education, salt intake levels, marital status, smoking and drinking behavior, body mass index, and exercise habits. Relative risk due to interaction and hazard ratios were calculated for additive and multiplicative interactions between the variables.
The results reported positive additive and multiplicative interactions between increasing air pollution levels and heatwaves, with hazard ratios and relative risk due to interaction being greater than 1 and 0, respectively. While the combined effect of heatwaves and receding green spaces on the incidence of hypertension was observed during additive and multiplicative interactions, the analysis did not detect any significant interaction between blue spaces and heatwaves.
The overall combined effect of the four variables — heatwaves, increasing PM2.5 concentrations, and the receding green and blue spaces — on the incidence of hypertension was seen to vary with respect to gender, age, and education level.
The authors discussed potential areas for changes in government policies and individual behavior to mitigate the increased risk of cardiovascular diseases due to environmental changes. While the emissions of PM2.5 must be controlled on a large-scale level, choosing landscape and building designs that improve cooling and ventilation, as well as increasing the number of publicly accessible green spaces, can decrease the risk of hypertension to some extent.
Overall, the results suggested that heatwaves, increasing air pollution, and receding green spaces had a combined effect in increasing the risk of hypertension, which varied according to age, sex, and education levels. Individual and government-level strategies to decrease air pollution and increase green expanses could have some benefits on cardiovascular health.
- Zhou, W., Wang, Q., Li, R., Aimulaguli Kadier, Wang, W., Zhou, F., & Ling, L. (2023). Combined effects of heatwaves and air pollution, green space and blue space on the incidence of hypertension: A national cohort study. Science of the Total Environment, 161560. https://doi.org/https://doi.org/10.1016/j.scitotenv.2023.161560
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