Pollen, particulate matter and SO2 in ambient air
Asthma and hay fever have both been rising dramatically in prevalence in Britain, as well as in other industrialized nations. In Cardiff, Wales, where the original research was conducted, 3.2% of primary school children reported hay fever in 1964; the figure rose to 12.7% by 1994. In 1973, 5.5% of 12-year-old children reported having asthma, which leaped to 12.0% by 1988. Obviously, something has changed, and the culprit most often under suspicion is the environment.
Two theories have been popular: that there is increased exposure to allergens in the home and more indoor and outdoor pollutants. However, recent studies, including this one, are disproving the implied connection. For instance, the increase in asthma has been blamed on a rise in the dust mite population at large, but a 10-year study showed no increase and, moreover, all social classes were exposed to very high concentrations of dust mites in the 1970s. However, the researchers concede that, due to changes in techniques for measuring allergens over 30 years, it is possible that dust mites are more abundant today.
Applying the same logic to hay fever, these researchers decided to look at the available data to determine whether there had been a rise in exposure to grass pollen over a period of 15 years. At the same time, they also looked at measurements of two air pollutants, sulphur dioxide and particulate matter (black smoke), because another theory holds that such pollutants increase sensitization to allergens. The original study had compared two groups of 12-year-old children, one in 1973 and the other in 1988, and calculated the incidence of asthma and hay fever (and eczema for good measure). The results showed that increases in reports of ever having allergic disease were significant: asthma more than doubled (5.5% to 12%), hay fever rose somewhat (9.4% to 14.9%), and eczema showed the largest increase of all (4.8% to 15.9%). But in this study, when these statistics were compared with the measurements of grass pollen and the air pollutants, the results were intriguing: there was no corresponding rise in grass pollen concentrations to account for the increase in hay fever, and the air pollution levels had been considerably higher in 1973. In fact, air pollution levels have been steadily decreasing since Britain implemented — and enforced — Clean Air Acts in 1956 and 1968.
What, then, can account for the increased prevalence of allergic diseases, if grass pollens and at least some air pollutants have been exonerated? While the researchers note that other pollutants or allergens not under investigation may be contributing to the increase, they think it more likely that there has been a shift in the population’s susceptibility to allergens, especially in childhood. They are looking at two theories to explain this. First, because Westernized countries have been highly successful in combatting early childhood infections, this recent change may be shifting the immune response from a resistant one to an atopic one that is very sensitive to allergens. Second, a significant dietary change may be responsible for breaching the lung’s defence system: there has been a marked drop in the British population’s intake of foods containing antioxidants. If this does turn out to be the case, as other researchers also believe, then a practical and effective method of combatting allergic disease can be implemented. After all, changing a diet to include fresh fruits and vegetables is more easily done than wiping out dust mites and air pollution.
Journal: Q J Med 1996; 89:279-84.
Author(s): Seaton A, Soutar A, Mullins J
1. Why were 12-year-old children selected for study and what inspired this survey to compare asthma and hay fever statistics?
The original epidemiological study was done in Cardiff in two surveys by Burr and colleagues in 1973 and 1988. In the U.K., age 12 is usually the last year spent in primary school, making it a convenient point of reference. Burr’s is one of the few studies done that shows conclusively that there has been a rise in allergic disease in the past 20 years. Our survey was done to obtain evidence with regard to the controversy going on as to whether the increase in asthma and other allergic diseases could be due to an increase in exposure to allergens, especially dust mites. Unfortunately, there are no suitable data for looking at mite content in houses. However, since the incidence of hay fever has also increased over the same period, we used this data for purposes of analogy. In hay fever, you can see if the increase is due to an increase in grass pollen. Since there was no rise in grass pollen, we may be able to infer that allergic diseases can increase without a corresponding rise in allergens. It’s likely that there’s been an increase in susceptibility to the allergen.
2. How do you account for the rise in asthma?
I’ve argued that the concurrent rise in asthma, hay fever and eczema is unlikely to have been induced by a change in the environment in terms of toxic insult or injury to the lungs, or eyes, nose, etc. This is because there have been no demonstrable rises in such factors as increased pollution, smoking during pregnancy and such like. Instead, there may be a change in response to environment rather than the environment itself. This was until recently a rather heretical view, but now seems to be gaining currency. The increase in susceptibility seems to go along with a Westernized lifestyle, as asthma is more prevalent in Western cultures. This may be due to diet, which may be lacking in important antioxidants, or it may be related to a change in the pattern of infections in childhood. The body’s defence reactions may be switched to an allergic type of reaction. For instance, there is evidence that if children have measles, they are less likely to have asthma.
3. Why are antioxidants important to lung function?
The hypothesis is that diet may be influencing the rise in allergic diseases, as a Western diet is less rich in fresh foods than 20 years ago. Damage to the airways occurs due to the release of free oxygen radicals from defensive cells, which become inflammatory. These cells release very active chemical radicals that are there to kill off bacteria. However, if you get an inflammatory reaction as a result of something other than bacteria, the free radicals released need to be mopped up before they do damage. This is usually done by ample availability of antioxidants, which are designed to stop free radicals from doing harm. This mechanism is supposed to control the body’s powerful defence system but can instead turn into a sort of “friendly fire.” Defences against friendly fire include antioxidants that we can only get through our diet, like Vitamin C, which the body can’t manufacture. If a diet is low in Vitamin C, it may be that your lungs become more vulnerable to attack by the free radicals released in response to dust mites, or pollen, or whatever.
To investigate whether the rise in allergic disease is explicable on the basis of an increase in the concentrations of allergen and urban air pollutants to which the population has been exposed, we compared the concentrations of grass pollen, sulphur dioxide and black smoke to which two samples of children with previously measured prevalence of hay fever had been exposed, in Cardiff, South Glamorgan. In these studies there had been a 59% increase in the prevalence of hay fever among 12-year-old school children between 1973 and 1988. Exposures to grass pollen of the two populations had been no different, but the earlier sample, with the lower prevalence of hay fever, had been exposed to substantially higher concentrations of ambient particulate matter and sulphur dioxide. The rise in prevalence of hay fever, occurring without a rise in grass pollen concentrations, supports the hypothesis that the population has become more susceptible to airborne allergen. This increased susceptibility is unlikely to be a consequence of concomitant exposure to sulphur dioxide and particulate air pollution.