There is considerable speculation about the reasons for the dramatically increased incidence and prevalence of childhood asthma in the UK – and in many other countries – over the past 20 years. Because seasonal fluctuations seem to have special influence, environmental factors are likely suspects. Pollutants created by the increased use of the car – sulphur dioxide, nitrogen dioxide and ozone – have been the subject of special scrutiny.
Ozone is generated by the influence of sunlight and other atmospheric conditions (such as high temperatures and low wind speed) on primary pollutants such as sulphur dioxide. Fluctuations in concentrations of air pollutants have been known to trigger asthma attacks in susceptible individuals, or may interact in a more complex way to increase airway hyperresponsiveness to viral or allergy triggers.
This English study recorded the attendance of all children under 16 years of age who arrived at the emergency department of the Hillingdon Hospital in west London between March 1992 and February 1993. The researchers investigated the association between concentrations of known pollutants and the rates of emergency visits for wheezing episodes in children.
The researchers compared information on daily concentrations of pollutants from reliable meteorological sources to the daily attendance of wheezy and non-asthmatic (control) children they saw. In all, 1025 children attended the emergency department with an acute wheezy episode. Very strong associations were seen with ozone concentrations; correlations also were found with sulphur dioxide and nitrogen dioxide.
The incidence of acute wheezy episodes differed significantly between seasons, with the highest occurrence during the autumn and winter. The incidence of acute wheezy episodes was found to have significant associations with all three air pollutants, but was most highly associated with ozone. Nitrogen dioxide was not found to have a significant effect on the incidence of wheezy episodes.
Previous studies relating ozone concentrations to increased incidence of asthma attacks have shown conflicting results. However, it is clear that pollutants interact with one another and with allergic or infective factors. This study showed that concentrations above critical levels (and for ozone, above and below optimal levels) are associated with an increased incidence of acute asthmatic episodes. Whether physiological, chemical or biometeorological explanations lie behind this association is open to speculation.
Questions – Answers:
1. Why does ozone have a particularly adverse effect on asthma in children? Does it have the same effect on adults?
The more ozone that is around, the greater the effect on lung function and respiratory health in general. Ozone has oxidant effects and has been used as an antiseptic. There’s speculation that it may aggravate respiratory inflammations caused by viral infections.
2. Ozone seems to have no adverse effect at what you describe as “optimal” levels, but exacerbates wheezing if it’s at higher or lower concentrations. Do you know why?
We were very surprised at this result of our observational study, which was prompted by the enormous controversy around air pollution and childhood asthma. We compared the pattern of attendance of children arriving at the emergency department of our hospital to measurements of pollutant concentrations, adjusting for all kinds of meteorological and seasonal factors. When we initially looked at the results, it looked as though there was an inverse relationship with ozone, that is, it seemed that the more ozone there was, the better it was for child health. But when we looked at the measurements with non-linear analysis, it turned out to be a U-shaped relationship, that remained constant regardless of the season. That is, patterns of attendance for childhood wheezing at the emergency room were higher at both low and high concentrations of ozone.
But why does this happen? We have a couple of theories. It may be that ground-level ozone at some critical concentration is somehow protective – which is almost heresy to say because no one has ever shown this before. However, we found a point at which a particular concentration of ozone saw the lowest incidence of visits to the hospital – which, by the way, is very close to Heathrow Airport – for acute wheezing. But we don’t know what the mechanism is that might be protecting the airway directly. Another hypothesis, one that I favour, is that perhaps ozone is just a marker for other pollutants in the atmosphere. We found when measuring other pollutants that when ozone was at its lowest concentration and at its peak, that’s when sulphur dioxide and nitrogen dioxide also peaked. So perhaps other pollutants, such as hydrocarbons and benzenes, are accumulating at ground level that are more directly related to child respiratory health.
But this idea is still very controversial and surprisingly little work is being done on the relationship of these pollutants to asthma.
3. Why is the incidence of childhood acute wheezy episodes greatest in autumn and winter?
There’s a lot of speculation on this centring around the conventional wisdom that in autumn and winter children return to school and are therefore exposed to viruses in these more crowded environments that trigger respiratory distress.
The problem with this theory is that if you look at the other cold months of the year, when kids are still confined together, in most parts of the world there is a pattern of reduction in the incidence of asthma. That a “herd immunity” may be prevailing over the viruses could be a factor, but why children are more susceptible to viruses during autumn and winter is still difficult to explain: is it that they are just crowded together, or are there other factors? I would speculate that perhaps environmental factors play quite a large role – there may be seasonal changes in the air that predispose children to respiratory ailments.