“Diesel traffic makes asthma worse”, was the headline of BBC News yesterday. It reports that if you are asthmatic, “a spot of Christmas shopping in a busy town centre may damage your health as well as your bank balance”.
The story is based on a study of 60 adults with asthma who had their lung capacity tested before and after they spent two hours on a busy street and then two hours strolling in a park. The results showed that walking on the busy, traffic-filled shopping street caused a significantly greater reduction in lung function compared with a walk in the park.
The lung function of asthmatics can deteriorate while exercising and there are many other factors that can cause the airways to constrict. Although it is quite plausible that the diesel fumes were making the sufferer’s asthma worse, it is equally possible, at least in part, that it could be caused by a hectic visit struggling around the bustling crowds of a busy shopping street compared with a gentle stroll in the park.
Where did the story come from?
Dr James McCreanor and colleagues carried out the study. They were from the National Heart and Lung Institute, Imperial College and Royal Brompton Hospital, London, the University of Medicine and Dentistry of New Jersey School of Public Health and the Karolinska Institute, Stockholm. The study was funded by the Health Effects Institute. It was published in the peer-reviewed The New England Journal of Medicine .
What kind of scientific study was this?
This was a randomised crossover study, to see what the effects of short-term exposure to diesel traffic were for 60 adults with asthma, half of whom had mild asthma and half had moderate asthma. All participants were non-smokers with stable asthma, free from infective exacerbations and none of the participants had asthma severe enough to need oral steroid treatment. Before the walking tests took place, the participants had a chemical provocation test. This looked at the responsiveness of the participant’s air passages to see how much of an irritant chemical was needed in order to produce a fall in lung function. Lung function was measured by changes in FEV1 – the maximum volume of air that can be breathed out in one second.
During the period of November through to March, each of the participants walked for two hours along a pre-defined route in Oxford Street, London (used by only diesel-powered buses and taxis), or traffic-free Hyde Park, London. Each exposure session was separated by a period of three weeks. On both routes, the participants walked at a steady pace for about 3.7miles (6km), with a 15-minute rest period every half an hour. Both FEV1 and FVC (the maximum volume that can be inhaled into the lungs) were measured immediately before, and each hour during the walk. Participants also reported any asthma symptoms during the walk.
In the five hours after the walk, lung function tests were repeated, including another chemical provocation test. The next morning, the tests were repeated again, and sputum samples were obtained to look for the presence of cells that would indicate that an asthmatic immune response had occurred. One week before and one week after the experiments, the participants had recorded their asthma symptoms and peak expiratory flow rates (a short sharp exhalation into a hand held meter after a deep breath), in order to check that there had been no difference in the participant’s asthma before they carried out either of the tests.
As the participants walked the routes, they also wore a monitor to measure their exposure to nitrogen dioxide prior to the tests. A system was used to measure the concentration of ultrafine particles and nitrogen dioxide present in the air at each of the exposure sites. The researchers used statistical analysis to look at the relationships between air pollutants and asthmatic outcomes.
What were the results of the study?
The researchers found that the concentration of ultrafine particles, carbon and nitrogen dioxide on Oxford Street was higher than in Hyde Park. There were no differences in severity of the participants’ asthma before either of the two exposures; however, following the walks they found there to be significant differences.
Although the participants had no symptoms, on average their lung function (measured by FEV1 and FVC) was reduced by a significantly greater amount after the walk in Oxford Street (FEV1 fell by 6.1%) compared with the walk in Hyde Park (FEV1 fell by 1.9%). This difference remained at all time points in the hours following the exposures. The observed effects were greater in those with moderate asthma compared with those with mild asthma. There was no difference in the chemical provocation testing results after either exposure, or in the concentration of the type of cells most commonly found to accumulate in the sputum after an in allergic asthmatic reaction.
What interpretations did the researchers draw from these results?
The authors conclude that a leisurely two-hour walk in an area surrounded by diesel-powered vehicles caused a significantly greater reduction in lung function compared with a walk in the park, although there was no difference in asthma symptoms. They also say that they found the most significant difference in air pollutants between the two locations to be that of ultrafine particles and carbon, which they say supports the notion that the adverse respiratory effects are largely attributable to very small particles that can be deeply inhaled into the lungs.
What does the NHS Knowledge Service make of this study?
This carefully conducted experiment showed that levels of traffic pollutants, such as ultrafine particles, are greater in a busy location like Oxford Street compared with the green environment of nearby Hyde Park. It demonstrated that objective measures of lung function in asthmatics were slightly poorer following traffic exposure. A few more points to consider:
- Although measurements of lung function were less after the Oxford Street exposure, it is important to note that the participants themselves did not observe any difference in symptoms between the exposure sites, or require any more asthma relief medication. The size of the reduction in lung function at one site compared with the other was relatively small.
- Two hours was a short exposure, and the effects of longer term exposure or if the participants had been involved in more demanding activities than walking, such as running or cycling, are not known. Results may also be different if the study had been repeated in summer months.
- It is not definite that it is diesel in particular that is the cause of the problem. Exposure to petrol vehicles was not tested.
- This study has only looked at adults with asthma (average age 32); we do not know whether the same effects would be seen in children with asthma, in those with other lung conditions such as chronic obstructive airways diseases, or in people with no restrictive lung disease at all.
- As it was impossible to blind the participants of this study, some bias may be introduced as they may have expected there to be more of a reduction in their lung function following a walk in the traffic-filled environment. However, more of a difference in the subjective reporting of asthma symptoms would be expected to be seen if this were the case.
- People with asthma are typically more prone to have exacerbations while they are exercising and many factors can cause constriction of the air passages including cold weather, environmental pollutants, viral illnesses and stress. Although it is quite plausible that the diesel fumes were making the sufferer’s asthma worse, it is possible that, at least in part, symptoms were caused by a hectic visit struggling around the bustling crowds of Oxford Street compared with a relaxed stroll in the park.
As the authors conclude, “without further study, we do not believe that these findings should deter most people with asthma from visiting or working in busy urban environments”.
Sir Muir Gray adds...
Everyone should try to keep their life as free from chemicals as possible, but complete freedom is impossible as we depend on some chemicals. People with an allergic condition have even more reason to be careful, but equally important is the need for people with chronic conditions to live life to the full and trust how they feel; if no symptoms develop there is no need to avoid the challenge of Oxford Street.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 6 December 2007
Links to the science
N Engl J Med 2007; 357:2348-2358.