Temperature modifies the association between air pollution and respiratory disease mortality in Cape Town, South Africa

Abstract

Background: Due to climate change, an increase of 3-4°C in ambient temperature is projected along the South African coast and 6-7°C inland during the next 80 years. Very few studies in Africa investigated effect modification by a temperature indicator (e.g. apparent temperature, Tapp) on the human health effects of air pollution. Methods: Respiratory disease (RD) mortality data (ICD10 J00-J99) were obtained from Statistics South Africa; after ethics approval. The study period was 1 January 2006 to 31 October 2015. NO2, SO2, PM10, temperature and relative humidity data were obtained from the City of Cape Town municipality. A case-crossover epidemiological study design was applied. The present study focused on lag0-1; as most other studies. Models were adjusted for public holidays and Tapp. Days with Tapp below and above/equal the 25th (12.6°C) and 75th percentiles (20.3°C) were classified as cold and warm days, respectively. Results: Of the 20376 RD deaths, approximately equal numbers were from females, males, 15-64 and 65+ year olds. The daily NO2, SO2, and PM10 levels were 16.0, 9.2, and 29.9 μm/m3, respectively. Daily air pollutant levels regularly exceeded the WHO guidelines. In general, none of the air pollutants had significant associations with RD mortality on cold days. The younger age group seemed to be more vulnerable to NO2 and PM10 on normal days compared to the elderly. On warm days, females were more susceptible to PM10, e.g. RR 1.065 95%CI 1.001-1.133 compared to the unstratified analyses RR 1.034 95%CI 0.991-1.078. On warm days, the 15-64 year old group were more vulnerable to NO2 and SO2. Conclusions: These results indicate that the risk of RD deaths due to ambient air pollution exposure is different on cold, normal and warm days in the Vaal Triangle Air Pollution Priority Area.