Thursday February 24 2011
The research did not look at cycling as a risk factor
Cycling is “one of the biggest triggers of heart attacks”, claimed the Daily Mail. The newspaper said that pollution and spending time in traffic can also bring on heart attacks for drivers and commuters.
The story is based on research that assessed triggers of non-fatal heart attacks. It found that air pollution posed a very small risk to individuals, but was one of the most common heart attack triggers due to the sheer number of people exposed to it every day. Traffic was estimated as the biggest trigger of heart attacks, although it is unclear whether this was due to factors such as pollution, stress or noise.
It is important to note that this study looked at triggers for heart attacks and not at the underlying causes or risk factors for heart disease, which usually develop over many years. The study also had design problems which mean its results need clarification with further research.
The best way to avoid a heart attack is to exercise, eat healthily, avoid smoking and drink alcohol in moderation. For most people, the benefits of regular exercise, such as cycling, outweigh the risk from air pollution.
Where did the story come from?
The study was carried out by researchers from Hasselt University and Katholieke Universiteit Leuven in Belgium, and the University of Basel, Switzerland. It was funded by the Flemish Scientific Fund and Belgian Science Policy programme. The study was published in the peer-reviewed medical journal The Lancet.
The Daily Mail’s claim that cycling to work is one of the biggest causes of heart attacks could be considered as misleading. The study found that exposure to traffic, as measured by time spent on the road in any form of transport, and physical exertion were among the biggest triggers for heart attacks in the general population, but cycling was not picked out as a particular trigger. Also, the study did not address the issue of how physical activities, such as cycling, can reduce the risk of coronary heart disease, the major underlying cause of heart attacks.
However, the Mail included comments from independent experts, who pointed out that it was difficult to disentangle different risks and stressed that the benefits of exercise outweighed the risks from air pollution for most people. The article also stated that the foundations for heart disease are laid down over many years.
Some newspapers did not clarify that pollution appears to be a trigger in at-risk people rather than a cause of heart attacks in low-risk individuals. In other words, pollution appears to be the factor that triggers a heart attack in people with cardiovascular problems due to pre-existing disease or preventable lifestyle factors. For example, the headline in the Daily Express suggested that air pollution from traffic may pose a danger to the heart. The Daily Telegraph, on the other hand, correctly explained the difference.
What kind of research was this?
This review and meta-analysis combined data from 36 studies on the possible triggers for heart attacks.
It looked at the importance of triggers both in the population and in individuals who engaged in certain behaviours or faced certain exposures. For example, it looked at how many heart attacks were triggered by cocaine in the general population and how commonly cocaine use was a trigger of heart attacks among cocaine users. The authors point out that while it is recognised that heart attacks can be triggered by various factors, such as stressful events, physical exertion, heavy meals or air pollution, the level of risk associated with each trigger is unclear.
They calculated the population attributable fraction (PAF), which estimates the proportion of heart attacks that could be avoided if a risk factor were removed. PAF can be a useful tool for showing the impact of a risk factor on public health as it takes into account the prevalence of particular risk factors in the population as well as the size of the risk in the people exposed to it.
What did the research involve?
The authors searched electronic databases to identify studies of triggers for non-fatal heart attacks conducted from 1960 to 2010. They defined trigger events as stimuli or activities occurring 1 hour to 10 days before the onset of a heart attack. They included all population-based or hospital-based case-control and case-crossover studies with sufficient information about the number of patients and triggers to calculate population-wide risk. When sourcing air pollution research, they selected studies that used particles in the air with a diameter of 2.5 µm or less as an indicator of air pollution (they also included studies on particulate diameter of 10 µm or less but assumed that 70% of the measured particles would be 2.5 µm or less).
Using standard statistical methods, the authors calculated the relative risk posed to individuals by each trigger and the PAF of each (the proportion of total heart attacks that they estimated could be prevented by removing the trigger). For triggers that were assessed in more than one study, they carried out a meta-analysis to pool the results.
What were the basic results?
The researchers identified 36 studies that investigated the individual risk of 13 triggers for heart attacks. The average age of the people in each study ranged from 44 to 72 years. The researchers identified six triggers that had been covered by more than one study, with air pollution being covered by 14 studies involving nearly 600,000 people. Within the total study population, exposure to the triggers ranged from 0.04% for cocaine use to 100% for air pollution.
Taking into account the prevalence of exposure to different triggers, the researchers calculated their PAF values (the proportion of total heart attacks that they estimated could be prevented by removing the trigger). These were, in order of magnitude:
- 7.4% for traffic exposure
- 6.2% for physical exertion
- 5% for alcohol
- 5% for coffee
- 4.8% for pollution
- 3.9% for negative emotions
- 3.1% for anger
- 2.7% for a heavy meal
- 2.4% for positive emotion
- 2.2% for sexual activity
- 0.9% for cocaine use
- 0.8% for marijuana smoking
- 0.6% for respiratory infections
How did the researchers interpret the results?
The researchers say that air pollution is an important trigger of heart attacks in the population, of similar magnitude to other well recognised triggers such as physical exertion, alcohol and coffee. This finding has considerable relevance to public health, they add.
This study looked for triggers associated with having a heart attack. Crucially, these short-term exposures should not be confused with the medical and lifestyle factors that can build up over years and make a person more vulnerable to heart disease and heart attacks. In other words, this study does not suggest that factors such as pollution necessarily damage the body in a way that makes a heart attack more likely.
This study has a number of limitations, some of which the authors noted:
- The results relating to traffic and air pollution need to be interpreted with caution. Commuting may expose an individual to air pollution, stress, noise or numerous other factors that were not considered by this study.
- The impact of air pollution is based on specific measurements of particulates in the air. However, depending on location and weather, pollution may differ in its chemical constituents and particle size.
- The researchers’ estimates of the proportion of heart attacks that could be prevented by removing certain triggers may be inaccurate. This is because the prevalence of different triggers within different populations may vary.
- The study only looked at non-fatal heart attacks. The assessed triggers may play a different role in heart attacks that kill.
- An individual’s exposure to pollution or traffic may be linked to factors that also raise their risk of having cardiovascular disease or a heart attack. For example, poorer people may be more likely to live in areas where they are regularly exposed to heavy traffic and to have a poorer diet, which can lead to heart problems.
- The authors did not take the quality of the studies into account when selecting them and did not give the studies quality scores, even though some triggers in some studies were defined less accurately than in others. This means that the researchers’ risk estimates are less robust.
- Some well-documented triggers of heart attacks were not included in the review, such as exposure to environmental tobacco smoke and outdoor temperature.
- Air pollution and road travel are widespread, and the risk associated with pollution may have been distorted by the fact that so many people are readily exposed to both.
While the results of this study are of interest and importance for public health, further research will be needed to clarify them.