Does low level air pollution impact on health?

There is substantial evidence that long-term exposure to air pollution causes increased mortality and cardiovascular and respiratory illness in cities with high air pollution (WHO, 2016). Globally, air pollution was estimated to have caused 4.2 million deaths in 2015, up from 3.5 million in 1990 (Cohen et al 2017).

Most data on these adverse health effects come from populations exposed to relatively high pollution levels in North American and European cities. But we know less about the risk to health at the lower concentrations of pollution that we typically see in Australian cities. For governments in Australia that are responsible for reviewing air pollution standards – to protect the health of the population – this is a critical policy question.

The main pollutants of concern in Australian cities are particulate matter less than 2.5 micrometres in diameter (PM2.5) and nitrogen dioxide (NO2), which mainly arise from combustion of fossil fuels, through traffic, power generation and wood-heaters. In collaboration with researchers from the NHMRC Centre for Air pollution, energy and health Research (CAR), the CAUL Hub has enhanced the estimates of individual exposure to these pollutants through sophisticated modelling techniques, helping improve our assessment of the relationship between air pollution and health. Our research has indicated that there is no threshold for health effects from air pollution, as adverse health effects continue to occur at the lower levels of pollution seen in Australian cities.

CAR and CAUL Hub researchers worked together to link data on individual exposure to air pollution to two large cohorts: the ‘45 and Up Study’ in Sydney, NSW and the Health in Men Study (HIMs) in Perth, Western Australia (funded by NHMRC).

Our estimates of exposure indicate that the annual average concentration of PM2.5 was around 4.5µg/m3 for both Sydney and Perth. In comparison cities included in the European Study of Cohorts for Air Pollution Effects had averages ranging from 7.7 – 31.0 µg/m3. (Beelen et al. 2014).

By analysing data from HIMS, we found that PM2.5 absorbance, a marker of black carbon (diesel) exposure, increased the risk of all-cause mortality by 12% per unit increase (adjusted hazard ratio (aHR) of 1.12, 95%CI:1.02–1.23). (Dirgawati et al, 2019). These findings suggest that at the lower levels of pollution observed in Perth, there was an important impact upon deaths.

In the 45 and Up Study, we observed a small non-significant increase (aHR 1.10, 95%CI:0.89-1.37) in hospitalisations for asthma associated with PM2.5 among the participants (Salimi et al. 2018). No association was observed for respiratory hospitalisations overall. We also looked at Western Sydney participants only and found no associations between exposure to air pollutants and hospitalisation for all respiratory diseases. The impact on respiratory disease is less clear from our study, but they are suggestive that air pollution may increase asthma hospitalisations by 10%, even at these lower exposures.

An important outcome of this work was improved methods of assessing exposure. These methods involved using satellite and monitored data and novel statistical techniques (Hanigan et al 2017; Knibbs et al, 2018) as well as the development of techniques for regular updates of these data. The satellite-based land use regression estimates of PM2.5 and NO2 that we developed are now available for use in other studies through Terrestrial Ecosystem Research Network’s (TERN) CoESRA Virtual Desktop. This will enable further research in the area to build upon our findings.

These studies have provided valuable data for the lower end of dose response curve, which has been a significant gap in knowledge about the impacts of air pollution on health. Evidence that there are health effects, even at low level of pollution, will be important for Government health and environment agencies when reviewing current standards for air quality.

A central part of this research has been the collaboration formed with researchers from CAR. This group has a long history in researching air pollution and health and were able to lend their considerable expertise to help us study the health impacts of air pollution in Sydney and Perth.

Featured image credit: Seb Zurcher via Unsplash

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