"Air pollution is linked to increased risk of developing an irregular heartbeat and blood clots in the lung," BBC News reports.
A large study found that short-term exposure to small particulate matter – a form of air pollution linked to cars and other sources – was linked to a raised risk of death from these conditions.
Small particulate matter is known to potentially be dangerous – because of its size (which can be 100 times thinner than a human hair), it can bypass the body's defences against foreign objects and affect the heart and the lungs.
Reassuringly, though, the study found no clear evidence of a link between air pollution and the risk of having a heart attack or stroke.
But the link found between irregular heartbeats (atrial fibrillation) and blood clots in the lungs (pulmonary embolism) is still a matter of concern. Both conditions can cause serious complications, especially in vulnerable people with a pre-existing health condition.
The study reinforces the fact that we should not become complacent about the health dangers posed by all forms of pollution.
The study was carried out by researchers from the London School of Hygiene and Tropical Medicine and was funded by the Department of Health.
BBC News' and The Guardian's coverage was fair and the BBC included useful comments from independent experts.
This study set out to explore the short-term impact of air pollution on cardiovascular disease. Using a case-crossover design, it analysed links between the information taken from three national databases on cardiovascular disease and short-term exposure to various types of air pollution.
The researchers say that high levels of some air pollutants are associated with an increased risk of cardiovascular outcomes, although the mechanism involved is uncertain.
The study aimed to further our understanding of these mechanisms by examining the relationship between air pollution and a range of cardiovascular events.
Over the period 2003-09, the researchers used data from three national databases on cardiovascular disease in England and Wales. These included:
Using the last two databases, researchers looked at a range of cardiovascular disease events, including heart attacks, all strokes, ischaemic heart disease, chronic ischaemic heart disease, pulmonary embolism, atrioventricular conduction disorders, arrhythmias, atrial fibrillation and heart failure.
Arrhythmia is an abnormality of the heart rhythm and can be dangerous. Atrial fibrillation is a type of arrhythmia where the heart beats very fast and irregularly. A pulmonary embolism is a blockage in the pulmonary artery, which is the blood vessel that carries blood from the heart to the lungs.
The researchers looked at some 400,000 heart attacks, more than 2 million emergency admissions for cardiovascular problems, and 600,000 deaths from cardiovascular disease.
Using data from the monitoring station nearest to where patients lived, they looked at the average levels of air pollutants over a period of five days. Pollutant effects were adjusted for the ambient air temperature – recorded by the UK Meteorological Office – and the day of the week.
The air pollutants included carbon monoxide, nitrogen dioxide, particulate matter (PM10 and PM2.5), sulphur dioxide and ozone. Particulate matter refers to usually invisible particles floating in the air. They can be large (up to 10 micrometers, or PM10) or small (up to 2.5 micrometers, or PM2.5).
In their analysis, the researchers used a case-crossover approach, where the day of each health event was the case and all other days within the same month were controls.
The researchers found that:
The researchers say their study found no clear evidence for the effects of pollution on STEMI heart attacks (the most serious type) and stroke, but it did for pulmonary embolism and irregular heartbeat. Lead researcher Dr Ai Milojevic told BBC News that the strongest link was in the over-75s and in women.
They concluded that although it is likely that air pollutants affect cardiovascular health in several different ways, the lack of effects on STEMI heart attacks and stroke suggests it may partly act through "non-thrombotic" pathways – in other words, not through blood clotting.
This was a large national study that looked in detail at links between people's short-term exposure to air pollutants and national records on hospital admissions for heart attack, emergency admissions for all cardiovascular problems, and deaths from cardiovascular disease.
The study had some limitations – for example, as the authors say, it did not include heart attacks that took place before hospital admission. It also used fixed monitoring sites, which may not accurately reflect personal exposure to air pollution.
For the public, the results of this study are probably confusing. That's because the researchers were interested in finding out which particular cardiovascular events are linked to pollution so that they could further understand the ways in which pollutants act on the cardiovascular system.
They found links between pollution and irregular heartbeat and blood clots in the lung – both of which can be dangerous – but not between pollution and heart attacks or strokes.
Most experts believe that pollution – especially small particulate matter – can have an effect on health. It is sensible to avoid areas with severe pollution, especially for those with chronic disease.
You can find updates on air pollution at Defra's UK air information resource pages and from Defra's freephone helpline on 0800 55 66 77. The helpline also offers health advice.
Read more advice about air pollution.