Opinion: What Makes PM2.5 The Most Important Factor In Efforts To Beat Air Pollution

The Global Burden of Diseases study identifies PM2.5 as the most significant air pollutant, responsible for more than 90% of all deaths attributed to toxic air. File image.

Source : PTI

Globally, nearly 8 million people lost their lives to breathing toxic air, according to the State of Global Air (SoGA) Report released in June this year. The figure was 2.1 million in India. Out of the global total, more than 700,000 were children under the age of five, with more than 70% of child deaths reported in Africa and Asia alone. These figures make air pollution the leading risk factor causing death, second only to blood pressure, and a more potent killer than tobacco and poor diet. Increasing cases of debilitating diseases, including cancer, diabetes and tuberculosis and those impacting heart, lungs, kidneys and even childbirth, are being attributed to this single culprit. Concealed in these statistics is an overwhelming factor — PM2.5! 

The Global Burden of Diseases study identifies PM2.5 as the most significant air pollutant, responsible for more than 90% of all deaths attributed to toxic air. In India alone, PM2.5 exposure was responsible for 7% of daily deaths in 10 cities, as stated in the SoGA Report. There can be no stronger corroboration of the public health emergency resulting from air pollution, dominantly PM2.5 exposure. 

Fine Particles Cause Greater Risk

Exposure, both chronic and acute, to particulate matter or the airborne, inhalable particles, has been a cause of grave health ailments. Based on the particle size and different health outcomes, separate limits have been defined for PM10 (particles smaller than 10 micrometres), fine mode PM2.5 (particles smaller than 2.5 micrometres), and ultrafine mode PM0.1 (particles smaller than 0.1 micrometres). Given the size difference, inhalation of slightly bigger PM10 affects the nasal cavity and upper respiratory tract. However, the smaller PM2.5 particles can go much deeper in the lungs and even penetrate the bloodstream, leading to greater health risks. 

While sources of these particulate matter pollutants are largely similar, construction and traffic dust contribute more to PM10 whereas PM2.5 is dominant in combustion sources, also making it a potent indoor pollutant. It is worth noting that other than being directly emitted, PM2.5 can be formed indirectly from gases through atmospheric reactions. Moreover, there is growing epidemiological evidence that establishes the greater health risks from breathing fine and even ultrafine particles that can translocate to all organs, penetrate deeper, and get absorbed in the bloodstream. This stresses the need for gauging health impacts based on exposure to finer air pollutants, and explains why all global health impact assessment models use PM2.5 as an indicator, as well as ozone and NO2. 

A more robust scientific enquiry into the lethality of breathing toxic air has generated the need for redefining limits deemed safe for human exposure. The World Health Organization (WHO) released air quality guidelines for the first time in 1987. Based on nuanced understanding of health risks of air pollution exposure, these guidelines were updated in 2005 and again in 2021. From time to time, scientific evidence from global cohort studies proves that exposure to air pollution at concentrations even lower than previously understood as safe can have damaging impacts on human health. These include six pollutants — PM10, PM2.5, ozone (O₃), nitrogen dioxide (NO₂) sulfur dioxide (SO₂), and carbon monoxide (CO). For PM2.5, the latest WHO guideline recommends annual average concentration not exceeding 5 µg/m3, substantially lower than the previous limit of 10 µg/m3. The redefined limits further signify the adverse health outcomes associated with fine mode particulate pollutants. In fact, with the new WHO limits, almost all nations fail to meet the benchmark that implies that even the smallest exposures to PM2.5 can be linked to increased hospitalisation from heart diseases and asthma. 

Mitigation Through Policy Response

Clearly, it is PM2.5 and not PM10 or total particulate matter that can be used as a concise indicator to gauge the health burden resulting from air pollution. In China, a nearly 48% decrease in PM2.5 concentrations achieved through government policies aiming at abatement of air pollution resulted in a 21% reduction in attributable deaths between 2013 and 2020. The reduced onus on healthcare compounds into significant economic benefits other than positively impacting the overall state of the environment and meeting policy goals. 

India, too, has designed a roadmap for achieving air quality goals, but it needs to be streamlined. The National Clean Air Programme 2019 mentions achieving 20-30% reductions, revised to 40%, in particulate matter concentrations by 2026. The focus is to attain clean air in 131 cities that fail to meet the PM10 standards. The programme, despite the ambitious targets focusing on air quality improvement, simply fails to address the health implications arising from PM2.5 pollution. Despite the indisputable and authentic scientific evidence on the health adversities of PM2.5 exposure, most concerted clean air action in India excludes it from its target. It can be argued that more scientific cohort studies from India are needed to provide an epidemiological impact assessment of PM2.5 exposure. But in no case can the immense global evidence establishing dose response to PM2.5 pollution be negated or overlooked. 

Air pollution is the greatest environmental and health challenge in our country today. To abate the pace of deteriorating air quality it is imperative that the policy strategies we choose to adopt, and tailor according to our socio-cultural conditions, do not truncate or ignore the science that elucidates the problem. To achieve the troika of improved air quality, reduced climate impacts, and sustainability goal 3 geared towards healthy lives and well-being of all, PM2.5 should not lose its salience as an important air pollutant that must be clearly targeted in all policy actions. 

Sachchida Nand Tripathi is Dean, Kotak School of Sustainability, IIT Kanpur. Disha Sharma is a Fellow at the Center of Policy Research and Governance.

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