Nearly a decade's worth of data collected across Singapore suggests increased concentrations of tiny particles in the air can trigger cardiac arrests, making the need to cut air pollution levels around the world even more urgent.

Researchers looked for particles at least 25 times smaller than the width of a human hair known as PM2.5 particles (for 2.5 micrometers in diameter). Their small size means they can be easily inhaled, and they've been linked to a host of health problems, including autoimmune diseases.

Here, pollution levels in Singapore were tracked against more than 18,000 reported cases of out-of-hospital cardiac arrest (OHCA) between July 2010 and December 2018. Through statistical analysis, 492 of the cases could be attributed to increases in PM2.5 concentrations.

"We have produced clear evidence of a short-term association of PM2.5 with out-of-hospital cardiac arrest, which is a catastrophic event that often results in sudden death," says epidemiologist Joel Aik, from the Duke–NUS Medical School at the National University of Singapore.

This is an observational study, meaning we can only speculate over the relationship between the pollution levels and the cardiac arrests. What's more, air pollution measurements taken at air quality stations can't be assumed to reflect individual exposure.

However, there's enough in the data to indicate it's a link worth exploring further. The data showed that daily PM2.5 concentrations averaged out at 18.44 micrograms per cubic meter. Testing hypothetical reductions in air pollution, the researchers found that a drop of 1 microgram per cubic meter correlated to an 8 percent reduction in heart attack events, while a drop of 3 micrograms per cubic meter saw a 30 percent reduction. Hypothetically, those reductions translate to 39 and 149 fewer heart attacks, respectively.

There was also a clear drop in the risk of having a heart attack 3 to 5 days after exposure to higher levels of pollution, suggesting that the effects are short term. The researchers say cleaning up city air could save lives and reduce strain on hospitals.

"These results make it clear that efforts to reduce the levels of air pollution particles in the 2.5 micrograms or lower range, and steps to protect against exposure to these particles, could play a part in reducing sudden cardiac arrests in Singapore's population, while also reducing the burden on health services," says Aik.

OHCAs have a typical survival rate of around 10 percent, much lower than the chances of surviving a heart attack in hospital. So it's no exaggeration to say that reducing the number of these cases saves lives. We can add it to the long list of reasons we ought to clean up our air.

While this link has been spotted before, in cities such as New York and Melbourne, Australia, results have been inconsistent with data collected in other places such as Denmark. These inconsistencies tend to arise at pollution concentrations below the World Health Organization's air quality guideline values, but research shows there is no 'safe' level of exposure for population heart health.

What is clear is that the majority of us are breathing in air of poor quality, which is thought to be responsible for millions of premature deaths in both urban and rural areas each year.

The team behind the new study wants to see more done to control air quality in places like Singapore. With everything from traffic congestion to wildfires playing a part, there are a lot of places to start making progress, including indoors.

"This study provides strong evidence for the impact of air quality on health and should stimulate policy and ground efforts to manage emissions from key sources that can lead to PM2.5 increases and prevent potential harm to public health," says Marcus Ong, a clinician-scientist from the Duke–NUS Medical School.

"New policy interventions, such as phasing out internal combustion engine vehicles, can help to reduce the dangers."

The research has been published in The Lancet Public Health.