Many types of air pollutants are harmful to human health. These include gases – such as carbon monoxide, nitrous oxides and sulphur dioxide – and particulates. Particulate matter refers to microscopic solid or liquid particles that are suspended in the air. These can be human-made, such as from traffic emissions and the burning of fossil fuels, or natural, such as from sea spray or mineral dust from soils. Particulate pollution has size designations: small particulate matter, known as PM2.5, refers to particles 2.5 microns or less in diameter, while coarse particulate matter, PM10, means particles 10 microns or less in diameter. PM10 can cause irritation to the eyes, nasal and upper respiratory passages. But PM2.5 is a greater health concern because the smaller particles can travel deep into the lungs, cross into the bloodstream and be transported to the brain and other organs. What are the health effects of exposure? Particulate matter is classified by the International Agency for Research on Cancer as a group one carcinogen – substances that are known to cause cancer in humans. Air pollution poses one of the greatest environmental risks to health, according to the World Health Organization. The WHO estimates that in 2019, outdoor air pollution caused 4.2m premature deaths worldwide. Of these, 37% were due to ischaemic heart disease and stroke, while 18% resulted from chronic obstructive pulmonary disease and 23% from acute lower respiratory infections. In a landmark 2020 ruling, a London coroner found that air pollution was a cause of the death of a nine-year-old girl, Ella Kissi-Debrah. The coroner said she had been exposed to nitrogen dioxide and particulate matter exceeding WHO guidelines. “Air pollution is harmful to health, full stop,” says Michele Goldman, the chief executive of Asthma Australia. “The small particulates are a major concern because they don’t get trapped in the hair in our nasal passages in our upper airways.” Sign up for Guardian Australia’s free morning and afternoon email newsletters for your daily news roundup Exposure to PM2.5 increases the risk of many conditions, including lung diseases such as cancer, chronic obstructive pulmonary disease (COPD) and pneumonia; cardiovascular disease; stroke and neurological diseases. “Short-term effects can manifest in the form of an exacerbation of existing symptoms, impaired lung function, and increased hospitalisation and mortality rates,” says Mark Brooke, chief executive of the Lung Foundation Australia. “Long-term exposure to air with a high concentration of pollutants may also increase the incidence of COPD.” Studies have shown that PM2.5 exposure during pregnancy is linked to adverse birth outcomes, including a greater risk of preterm births and newborns with low birth weights. Because children breathe more rapidly than adults, they are exposed to greater air pollution. Research from the US has found links between air pollution from coal power plants and reduced test scores for children. People with chronic diseases are more susceptible to the effects of air pollution, which has also been linked to an increased risk for dementia. What is the safe level of PM2.5 exposure? Experts agree that there is no safe level of exposure to air pollution. The WHO’s global air quality guidelines recommend limiting PM2.5 air pollution to an average of five micrograms a cubic metre of air (5µg/m3) annually. The safe daily mean PM2.5 target should be less than 15μg/m3, the WHO says. The Australian national ambient air quality standards are higher, at 8µg/m3 averaged annually, and 25 µg/m3 averaged over a 24-hour period. “Defining a precise threshold [above which there are health risks] is difficult,” says Matthew Peters, a professor of respiratory medicine at Macquarie University. USesearch has found that a 10μg/m3 increase in two-day averaged PM2.5 concentrations increased emergency hospital admissions by 2.25% for heart attacks, 2.07% for respiratory conditions and 1.89% for cardiovascular disease. Another study found that two days of PM2.5 being increased by 10μg/m3 raised non-accidental deaths by 0.74%. To put this into perspective, PM2.5 levels in Melbourne exceeded 200μg/m3 on five days in the summer of the 2019-20, as a result of bushfire smoke. In the longer term, a recent study of nearly 33,000 people in four countries, published in the journal Nature, found that particulate matter promoted lung cancer-specific genetic mutations which increased the progression of tumours. Peters, who was not involved in the research, says: “the equivalent of three years of exposure – only three years – is enough to get a clearly measurable signal. That should be a concern. “If you think about kids – if the burden of genetic risk accrues to kids between the ages of two and five, they’re carrying this for the rest of their childhood and adult lives.”