Pollution 'linked to heart attack risk'

"Traffic fumes can trigger heart attacks, say researchers," The Guardian reported today. It said that "breathing in large amounts of traffic fumes can trigger a heart attack up to six hours after exposure".

This large study investigated the relationship between the risk of having a heart attack and exposure to different traffic pollutants. Researchers analysed nearly 80,000 heart attacks and the person's exposure to air pollution in the time leading up to the attack. Certain pollutants were found to be associated with an increased risk of a heart attack within six hours of exposure. After that time there was no increase in risk.

Importantly, as the increase in risk was only short term, the authors suggest that these heart attacks would have happened anyway and that pollution only made them happen earlier. In other words, the study does not appear to show that pollution triggers heart attacks in previously healthy people. It suggests that these attacks were in people already at risk.

This large, complex study is a valuable contribution to this area of research. Previous studies have found a link between pollution and risk of death, especially death from cardiovascular disease, but few have looked at the effects of exposure in the hours leading up to a heart attack.

People who have been diagnosed with heart disease and other conditions are currently advised to avoid spending long periods in areas with high traffic pollution levels.

Where did the story come from?

The study was carried out by researchers from the London School of Hygiene and Tropical Medicine. It was funded by the British Heart Foundation and the Garfield Weston Foundation. The study was published in the peer-reviewed British Medical Journal, along with an editorial discussing the study’s findings.

The study was widely reported in the press, which correctly reported that the increased risk was limited to the first six hours following exposure to pollution. Most reports also mentioned that the increase in risk was relatively small, and that pollution probably hastens rather than causes heart attacks.

What kind of research was this?

This was a case crossover study aimed at investigating possible short-term associations between air pollution levels and the risk of heart attack. This type of case control study design is often used by researchers trying to estimate the risk of transient, short-term events (such as pollution levels) on the risk of acute illness (such as heart attack). The cases, on the day of heart attack, act as their own controls for days when they did not suffer a heart attack.

The researchers point out that while several studies have shown associations between short-term exposure to common environmental pollutants and an increase in deaths from cardiovascular disease, the relationship between pollution and heart attacks is less clear. Their aim was to look at the effects of hourly exposure to air pollutants on the risk of heart attack.

What did the research involve?

The researchers used clinical data from a national register that records all hospital admissions for heart attack (defined as myocardial infarction and other acute coronary syndromes) in England and Wales. They reviewed 79,288 such diagnoses over the period 2003 to 2006 in patients residing in 15 cities.

Pollution levels were obtained from a national air quality database, which gets its data from urban background monitoring stations. For each city, they also obtained hourly levels of the following air pollutants: pollutant particles (PM10– the 10 denoting the size of the particles), ozone, carbon monoxide (CO), nitrogen dioxide (NO2) and sulphur dioxide (SO2). The researchers also obtained information on other factors that might influence risk of heart attack, including daily average temperature and humidity from weather monitoring stations and levels of certain viral infections, such as flu, from daily counts of laboratory confirmed cases.

For each individual heart attack the researchers collected hourly levels of pollution exposure for the day of the heart attack, which is called the “case” day, using the patient’s address. They then compared the person’s exposure to pollution on the case day with other days when they hadn’t had a heart attack. To do this, they looked at hourly pollution levels on a set of “control” days comprising every other day in the month the heart attack occurred.

Validated statistical methods were used to give a detailed assessment of whether there was an increased risk of heart attack per 10µg/m³ increase in pollution levels. The results were adjusted for other factors that might affect the risk of heart attack, including temperature, humidity, levels of certain viruses, holidays and season of the year.

The possible effect of pollution was investigated within five different timeframes before the heart attack had occurred – 1-6 hours, 7-12 hours, 13-18 hours, 19-24 hours and 25-72 hours. The researchers analysed each pollutant for its effect, both separately and combined with other pollutants.

They also analysed the data in different ways, looking at the possible modifying effect of different factors, such as age, smoking status, season and hourly temperature.

What were the basic results?

The researchers found the following results:

• When they looked at each separate pollutant, pollutant particles and nitrogen dioxide levels were associated with a very short-term increase in risk of myocardial infarction 1–6 hours later. The risk increased by 1.2% for this period (95% confidence interval 0.3 to 2.1).
• The risk increased by 1.1% (0.3 to 1.8) for every 10 micrograms of pollutant per square metre.
• When they looked at all pollutants combined, the effects persisted.
• Following the six-hour period where risk was increased, risk reduced, so that 72 hours after exposure there was no overall risk increase.
• There was no evidence of any excess risk associated with the five pollutants studied over a 72-hour period after exposure.

The researchers note that for nitrogen dioxide in particular the effect was greater among older people and those with prior coronary heart disease.

How did the researchers interpret the results?

The researchers say that higher markers of pollutant particles and nitrogen dioxide, which are typically markers of traffic-related pollution, seem to be associated with a temporary increased risk of heart attack 1-6 hours after exposure. However, they say the fact that risk lowered again six hours after exposure suggests that air pollution may be associated with hastening heart attacks in people that were going to have them anyway (called short-term displacement), rather than increasing overall risk.

They say that pollution may trigger heart attacks by various mechanisms, such as increased inflammation, increased blood “stickiness” or rises in blood pressure. They say that the effect of air pollution on deaths from heart and respiratory problems is an established one, but that pollution may not directly increase the immediate risk of heart attack, but may increase risk through another mechanism. However, they add that this finding should not undermine calls for action on air pollution, which has clear associations with increases in respiratory and cardiovascular mortality.

Conclusion

This is an impressive and well-conducted study, but as the authors note it has limitations, including the following:

• The authors say that they did not have sufficient data to look at heart risk from fine particle pollution called PM2.5 (where the particles are much smaller than PM10 particles).
• Pollution measures taken at fixed external measurement sites may not reflect fluctuations in personal exposure indoors and may result in some degree of measurement error.
• Adjusting the analyses for confounding factors such as temperature may have reduced the statistical power of the study.
• Heart attacks were only recorded if they led to hospital admission. There may have been some heart attacks (especially fatal ones) that took place outside hospital that might have influenced the results.

In conclusion, these findings support the idea that pollution may trigger heart attacks in people who are already vulnerable, but that pollution itself does not increase the overall risk. Current advice for elderly and vulnerable people is to avoid long periods in areas of high pollution, such as busy roads.