It’s no secret that air pollution is a serious problem facing the world today. Just how serious? A new study on global daily levels of air pollution shows that hardly anywhere on Earth is safe from unhealthy air.

About 99.82% of the global land area is exposed to levels of particulate matter 2.5 (PM2.5) — tiny particles in the air that scientists have linked to lung cancer and heart disease — above the safety limit recommended by the Word Health Organization, according to the peer-reviewed study published Monday in Lancet Planetary Health. And only 0.001% of the world’s population breathes in air that is considered acceptable, the paper says.

Conducted by scientists in Australia and China, the study found that on the global level, more than 70% of days in 2019 had daily PM2.5 concentrations exceeding 15 micrograms of gaseous pollutant per cubic meter — the WHO recommended daily limit. Air quality is particularly worrisome in regions such as southern Asia and eastern Asia, where more than 90% of days had PM2.5 concentrations above the 15 microgram threshold.

While any amount of PM 2.5 is harmful, scientists and regulators are typically less concerned about daily levels than they are about chronic exposure.

“I hope our study can change the minds of scientists and policymakers for the daily PM2.5 exposure,” said Yuming Guo, the lead researcher and an environmental health professor at Monash University. “Short-term exposure, particularly sudden increase, to PM2.5 has significant health problems … If we can make every day with clean air, of course the long-term exposure of air pollution would be improved.”

While scientists and public health officials have long been at alert to the dangers — air pollution kills 6.7-million people a year, with nearly two-thirds of the premature deaths caused by fine particulate matter — quantifying the global exposure to PM2.5 was a challenge due to a lack of pollution monitoring stations.

Guo and his coauthors overcame that challenge by marrying ground-based air pollution measurements collected from more than 5,000 monitoring stations worldwide with machine learning simulations, meteorological data and geographical factors to estimate global daily PM2.5 concentrations.

When it came to estimating annual exposure across all regions, the researchers found that the highest concentrations occurred in eastern Asia (50 micrograms per cubic meter), followed by southern Asia (37 micrograms) and northern Africa (30 micrograms). Residents of Australia and New Zealand faced the least threat from fine particulate matter, while other regions in Oceania and southern America were also among the places with the lowest annual PM2.5 concentrations.

They also examined how air pollution changed over the two decades up to 2019. For instance, most areas in Asia, northern and sub-Saharan Africa, Oceania, and Latin America and the Caribbean experienced an increase in PM2.5 concentrations over the 20 years, driven in part by intensified wildfires. Annual PM2.5 concentrations and high PM2.5 days in Europe and northern America decreased, thanks to stricter regulations. Fine particulate matter is made up of soot from vehicles, smoke and ash from wildfires and biomass cook-stove pollution, plus sulfate aerosols from power generation and desert dust.

The article also points out how levels of fine particulate matter vary depending on the season, a reflection of human activities that accelerate air pollution. For instance, northeast China and north India recorded higher PM 2.5 concentrations from December to February, likely linked to an increased use of fossil fuel-burning heat generators during the winter months. South American countries such as Brazil, on the other hand, had increased concentrations between August and September, probably connected to slash-and-burn cultivation in the summer.