Medical Matters: Cyclists like myself often fear that we inhale the contents of stationary exhausts as we speed by a line of traffic. I see cyclists wearing face masks in traffic which makes sense as respiratory problems are well documented.
Exhaust fumes are complex but mainly comprise particulate matter and benzene-related chemicals. Research evidence from Denmark and England show that concentrations of particles and benzene products were two to four times greater in car cabins than in cyclists' travel zones.
Concentrations are affected by the direction of the wind, and the further away the cycle path is from the traffic, the less pollutants inhaled.
Engineers in Trinity College Dublin have compared cyclists' exposure to pollutants with lower deck bus travel and the bus was slightly safer in central Dublin.
The American Heart Association confirm that short- and long-term exposure to air pollution is associated with increased heart disease. The World Health Organisation estimates that 800,000 people die prematurely due to air pollution each year. Beijing is currently trying to clean its air in advance of the Olympics based on this and other evidence.
In a recent experiment conducted by researchers in Edinburgh and Sweden and reported in the US New England Journal of Medicine, we all have more pressing food for thought.
They took 20 men who had a previous heart attack and exposed them to one-hour periods of dilute diesel exhaust or clean-filtered air.
During the exposure the men did 15-minute periods of moderate exercise on an exercise bike. Most of the men had disease in a single artery around their hearts.
They did a series of ECGs and blood tests throughout the study and at six hours afterwards. The interesting and important findings relate to the ECG and to the blood tests for thrombosis or clots.
An ECG measures the electrical activity of the heart and when it is under pressure from lack of oxygen from good blood flow, certain well-defined patterns occur. In this study they are called ST depression.
These ECG changes occurred in both exhaust and filtered air but worryingly the ST depression was much greater in the diesel group.
Clot formation is the end result of a series of complex processes and the body manufactures its own clot buster.
This is an enzyme called a plasminogen activator which, if reduced in activity, allows a clot to form in the artery and results in blocking the blood supply. This blockage in the heart causes a heart attack.
In this study the clot-busting enzyme plasminogen was depressed even six hours after the diesel fumes were turned off.
Thus brief exposure to diluted diesel fumes caused a reduction of blood supply to the heart and a reduction in life-saving clot-busting enzyme in the blood.
Scientists do not know if the particulate matter or the benzenes are the culprits but the money seems to be on the particles at the moment.
What are the implications of all this?
It is always wise to give research time to be examined by other scientists and for good quality advice to emerge. It is important to remember the study subjects were engaged in moderate exercise and not sitting passively. However, if this research holds up to further scrutiny and is seen to adversely affect the already diseased heart, both doctors and politicians will need to act.
It is important to know what is causing the damage as the addition of what is called a particle trap to exhausts could help reduce risk to the heart.
Based on what we know now it seems that if you have known cardiac disease and take regular moderate exercise by walking vigorously, jogging or cycling, you are best to exercise well away from traffic.
The Trinity College engineers found in their study that levels of pollutants in playing pitches surrounded by traffic could be higher than inhaled pollutants for cyclists or bus travellers.
It would seem wise to take steps to improve the blood supply to your heart. Taking a puff of your glyceryl trinitrite would certainly help as this dilates the blood vessels which in turn increase the blood supply to the heart.
What about car drivers with established cardiac disease sitting in traffic?
We don't know but if you habitually get agitated and your pulse goes up, it may be wise to calm down. Getting good travel advice on where the hold-ups are and trying to avoid times of peak traffic are other tips.
The Stern report on climate change says we can avoid the worst impacts of climate change if we act now. This means a more specific understanding of how our environment affects our health.
To do this we need to earmark money for research to identify the dirty air spots we use every day and to develop techniques to clean them up.
It is a truly onerous and life-saving time to be a Green Minister in Government.
• Tom O'Dowd is professor of general practice, Department of Primary Care and Public Health, Trinity College Dublin.