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Response to Dr Down - Shop floor epidemiology

Down asks whether there is any research linking roads and respiratory health. There is. Edwards and colleagues found that children admitted to hospital with asthma were more likely to live in an area of Birmingham with a high traffic flow than were control children [1]. Elliott and colleagues, by contrast, found that asthmatic cases, defined as needing regular medication, were no more likely to live by main roads than were nonasthmatic controls [2]. Elliott and colleagues see these findings as contradictory, but perhaps asthma prevalence is unaffected by the roads and asthma severity is. Waldron and colleagues found that the prevalence of asthma in East Surrey was lower near the M25 than further away [3].

Proxy measures

Roads, of course, are a proxy for the amount of air pollution to which people are exposed. Schwartz found that more cases of croup, but not bronchiolitis presented to hospital when the level of nitrogen dioxide and particulates was higher [4]. It is hard to disentangle weather effects from this as weather is a major influence on the concentration of outdoor air pollution. The study cannot determine whether incidence or severity is affected.

Study findings

A Finnish study found that upper respiratory infections (URTIs) of all types were more common in toddlers in a polluted city than in two cleaner cities: odds ratio (95% confidence intervals) were 2.0 (1.3-3.2) [5]. Toddlers in the most polluted parts of the polluted city had an odds ratio of 2.0 (1.0-4.0) for URTIs, compared to those in the less polluted parts. A study in Japan found that allergic rhinitis was more common in polluted areas and in areas with high concentrations of cedar pollen, but was most common in areas with high pollution and high cedar pollen levels [6].

In summary, we know that pollution from vehicles can exacerbate respiratory conditions, but we do not know whether it also affects the incidence, although the Japanese results suggest it might.

What Dr Down could do

It would be hard for Dr Down to study this problem in his/her practice. S/He could do a time series study like Schwartz. If s/he had access to daily pollution measures, s/he could examine the number of consultations in relation to the pollution level: a tricky statistical problem. Another approach is to compare his/her practice to one or more control practices away from the motorway to see if the number of consultations is different.

The difficulty here is knowing whether the differences in consultation rates are due to the pollution, or some other characteristic of the patients or the practice style. Studies of the effects of air pollution usually require large numbers of people and are best done using a population sample, rather than a sample of consulters.

[Mystery note: Bandolier apologises for losing the name and address of the reader who wrote this piece. Please let us know so we can acknowledge you properly]

References

  1. Edwards J, Walters S, Griffiths RK. Hospital admissions for asthma in preschool children: relationship to major roads in Birmingham, United Kingdom. Arch Environ Health 1994;49:223-7.
  2. Livingstone AE, Shaddick G, Grundy C, Elliott P. Do people living near inner city main roads have more asthma needing treatment? Case control study. Br Med J 1996;312:676-7.
  3. Waldron G, Pottle B, Dod J. Asthma and the motorways - one District's experience. J Publ Hlth Med 1995;17:85-9.
  4. Schwartz J, Spix C, Wichmann HE, Malin E. Air pollution and acute respiratory illness in five German communities. Environ Res 1991;56:1-14.
  5. Jaakkola JJ, Paunio M, Virtanen M, Heinonen OP. Low-level air pollution and upper respiratory infections in children. Am J Public Health 1991;81:1060-3.
  6. Ishizaki T, Koizumi K, Ikemori R, Ishiyama Y, Kushibiki E. Studies of prevalence of Japanese cedar pollinosis among the residents in a densely cultivated area. Ann Allergy 1987;58:265-70.



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