Should World Health Organisation air pollution limits be universal?

No two regions of the globe are the same, neither is their particulate matter, leading scientists to call for guidance that better reflects regional differences. 

When the World Health Organisation released its new recommendations for maximum exposure to particulate matter PM2.5 last autumn, cutting previous safe limits by 50% – from 10 to five micrograms per meter cubed – it left many policymakers scrabbling to keep up. Simply put, if the old limits were already being exceeded many times over, the chances of falling in line with the renewed advice was slim to none. 

Nevertheless, a number of countries are already looking to change their own targets in order to match those set by WHO. The overall aim being to cut back on emissions from transport, industry, agriculture and other sources to bring air pollution down to safe levels, helping combat both the rapidly spiralling climate and health crises linked to air pollution. But now a new study undertaken by MIT Department of Civil and Environmental Engineering suggests that even if humans stop emitting noxious fumes altogether, many parts of the globe will still fail to meet PM2.5 limits. 

Published this week in the journal Environmental Science and Technology Letters, the research shows that if all anthropogenic emissions stopped, 50% of the world’s population would still be exposed to PM2.5 concentrations that exceed WHO guidelines, and are deemed unsafe for humans. Particulate matter including dust, sea salt, and organics from vegetation would still exist in the atmosphere, and pose a health risk. 

‘If you live in parts of India or northern Africa that are exposed to large amounts of fine dust, it can be challenging to reduce PM2.5 exposures below the new guideline,’ said Sidhant Pai, co-lead author and graduate student. ‘This study challenges us to rethink the value of different emissions abatement controls across different regions and suggests the need for a new generation of air quality metrics that can enable targeted decision-making.’

In order to conduct the work, the team developed a series of model simulations to explore the viability of meeting the new guidelines globally under different emission reduction scenarios with 2019 was used as the representative baseline year. In some cases, one source of PM2.5 emissions was turned off, in others all were, painting clear pictures of how much particulate matter would remain in the atmosphere from natural sources and fires. The chemical composition of particulates was also analysed, allowing experts to understand main sources for each global region. 

In the Amazon, for example, PM2.5 is highly likely to consist of carbon-containing aerosols linked to deforestation fires. In Northern Europe, meanwhile, there are greater ties to vehicles and fertilisers. According to the MIT team, this confirms policies for two regions cannot be the same if there is a chance of succeeding in the worldwide goal of bringing particulate pollution down. The study can also be seen as a starting point for further research into different health implications of types of PM2.5, presenting an opportunity to expand on guidelines to ensure they are fit for purpose. 

Image credit: Mohmed Nazeeh