The rising temperatures that come with climate change present a global health challenge. There is scientific evidence that air pollution and hot weather increase the risk of cardiovascular disease hospital admissions - and may even end in death. Heart attacks, heart failure and abnormal heart rhythms are all examples of cardiovascular diseases. Evidence of the interaction between temperature and air pollution - and the combined effects on people's health - is also growing.
Air pollution and temperature seem to contribute to the same health effects, as various studies and reviews have found. These two environmental exposures act on people's bodies in similar ways. Exposure to particles in the air is linked to inflammation and increased risk of blood clotting. Heat may also enhance clotting and raise cholesterol levels. Our bodies' systems function more slowly in lower temperatures, which may make air pollution less dangerous on colder days.
But until now, the interaction of temperature and air pollution and its contribution to these diseases has not been studied conclusively in South Africa - or anywhere on the African continent. Our study is the first of its kind in Africa. We looked at the relationship between temperature, air pollution and hospital admissions for cardiovascular disease in Cape Town - where data quality was good.
What we found
Our study included 54,818 hospital admissions during the period January 2011 to October 2016.
The air pollutants we included in the study were nitrogen dioxide, sulphur dioxide and what is known as PM₁₀ - tiny particles made up of soot and other chemicals in the air. Sources of air pollution in the city include traffic, industry, residential wood/coal burning, refuse burning, dust from unpaved roads and occasional seasonal wild fires.
The daily PM₁₀ levels exceeded the daily World Health Organization (WHO) air quality guideline on 90 days of the 2,131 days. The daily sulphur dioxide levels were higher than the daily WHO guideline on 56 days. The mean level of PM₁₀ during the study period was higher than those of some European cities, like Amsterdam or Copenhagen, but lower than those of other cities around the world, like Moscow, Cairo and Hong Kong.
We also looked at the apparent temperature (Tapp) during the same period. Tapp reflects the physiological experience of combined exposure to humidity and temperature - it is a better indicator of what the temperature feels like.
The mean Tapp was 16.3C. We defined warm days as Tapp levels above 20.3C (the 75th percentile of the Tapp levels) and cold days as below 12.3C (the 25th percentile of the Tapp levels). This approach was similar to that of other studies. Over the 2,131 days of the study, 529 were warm and 546 were cold. The sources and chemical composition of air pollution may vary with weather indicators such as Tapp. In this study PM₁₀ and Tapp levels were positively correlated - the air contained more of these particles on warmer days. Nitrogen dioxide and sulphur dioxide had negative correlations with Tapp, meaning higher levels on colder days.
We found that the risk of hospital admission for cardiovascular disease was not the same for everyone. Tapp also made a difference. Normal Tapp levels (between 12.3C and 20.3C) were not associated with higher hospital admissions.
When air pollution levels were higher - on warm or cold days - people aged 15-64 years were the group most likely to be admitted to hospital for cardiovascular disease. In this age group, admissions increased with PM₁₀ levels, with a greater risk on warm days than on cold days. This may be because people in this age group are more active outdoors than older people and are exposed to outdoor temperatures and air pollution simultaneously.
In our study, females appeared to be more at risk than males when PM₁₀ levels were raised. In contrast, males were more vulnerable than females to the effects of nitrogen dioxide and sulphur dioxide.
What should be done
One of the limitations of our study was that we used electronic patient records from private hospitals, which serve a small fraction of the population. Currently, electronic data are not readily available from public hospitals. We also investigated only one city in South Africa. The results cannot be applied to the entire population of the country.
Another limitation was that we did not have data for another pollutant, PM₂.₅. This pollutant is reported to be more toxic than PM₁₀ as it penetrates deeper into the lungs.
The results have implications for public health strategies to prevent the risk of hospital admissions for cardiovascular disease. These strategies may include early warning systems especially targeted for vulnerable population subgroups.
Author: Janine Wichmann - Associate Professor, University of Pretoria