How air pollution affects health: what science knows
These findings come from the World Health Organization and from studies in peer-reviewed journals. They describe what happens across whole populations over months and years, and do not say what one day's air will do to any one person.
Fine particles reach the blood
Particulate matter can travel deep into the lungs, and the finest particles can pass into the bloodstream. Both short spells and long stretches of exposure are linked to illness and death from heart and lung disease. Long-term exposure is also linked to lung cancer and to harm during pregnancy and at birth.
Source: WHO, Types of pollutants
No level is known to be safe
WHO says some pollutants cause harm at any level. In 2021 it cut its guideline for the yearly average of PM2.5 to 5 micrograms per cubic metre of air.
A study of ten Indian cities found that deaths rose about twice as steeply with PM2.5 when only days below India's own daily standard, 60 micrograms per cubic metre, were counted.
The figures elsewhere on this site are CPCB's AQI scores on a 0 to 500 scale, not amounts in micrograms like these.
Sources: WHO, Types of pollutants; WHO global air quality guidelines (2021); de Bont et al., Lancet Planetary Health (2024)
A cause of lung cancer
In 2013 WHO's International Agency for Research on Cancer classified outdoor air pollution, and separately the particles in it, as causes of cancer in humans, finding sufficient evidence that they cause lung cancer.
Source: IARC press release 221 (2013)
Deaths worldwide
WHO estimates that outdoor air pollution caused 4.2 million early deaths around the world in 2019. About 68% were from heart disease and stroke, 14% from chronic obstructive pulmonary disease, 14% from lower respiratory infections and 4% from lung cancer.
In 2019, 99% of the world's people lived where the air did not meet WHO's guidelines.
Deaths in India
A 2021 study in Lancet Planetary Health estimated that air pollution, counting outdoor particles, smoke from household fuels and ozone, was linked to 1.67 million deaths in India in 2019, nearly 18% of all deaths. It put the economic loss at 1.36% of India's GDP.
These are estimates of deaths linked to pollution across the country, not a count of people it killed.
Source: India State-Level Disease Burden Initiative, Lancet Planetary Health (2021)
Daily deaths in Delhi and other cities
A 2024 study in Lancet Planetary Health followed ten Indian cities, including Delhi, Mumbai, Kolkata and Chennai, from 2008 to 2019. Each rise of 10 micrograms per cubic metre in the two-day average of PM2.5 came with 1.4% more deaths that day.
Across the ten cities, PM2.5 above WHO's guideline was linked to 7.2% of daily deaths, about 33,600 a year. Delhi had the highest share: 11.5%, about 12,000 deaths a year.
The study's figures are averages over 12 years. A single day's reading cannot be turned into a death count.
Ozone and nitrogen dioxide
Too much ozone can cause breathing problems, trigger asthma, reduce how well the lungs work and lead to lung disease. Nitrogen dioxide irritates the airways and makes respiratory disease worse.
Source: WHO, Types of pollutants
For advice about your own health, talk to a doctor.
Compiled by the HT Pollution Dashboard from the sources linked with each finding.