These days, asthma and its causes are fairly well understood. Inflamed lungs, reversible airway obstruction, and hyperresponsiveness of airways are all typical symptoms of asthma. These can be triggered by a growing list of risk factors, especially from air pollutants and airborne allergens. Even so, in spite of a better grasp of the disease and improved therapy, asthma mortality is climbing. Researchers do not yet have clear answers to some important questions, including why death rates have been going up for the last 10 years, and why the incidence is higher among blacks than whites.
The environment generates countless allergens and irritants, but sorting out which ones have an impact on asthma has important implications for treatment, especially as avoidance is one therapeutic option. The authors of this paper scrutinize environmentally induced asthma from three perspectives: by examining the role of air pollutants such as sulphur dioxide, nitrogen dioxide, and airborne acids; by looking at ways of controlling airborne allergens such as cat dander and dust mite; and by exploring the possibility that interactions between air pollutants and allergens may be responsible for additional detrimental effects on respiratory health.
Air Pollution
Controlled studies show that asthmatics are particularly sensitive to sulphur dioxide. Ordinary people can inhale as much as 5.0 parts per million of sulphur dioxide and experience only slight reductions in airway function. For asthmatics, as little as five minutes of sustained exposure to sulphur dioxide at concentrations of 1.0 parts per million or less produces pronounced bronchoconstriction. Elevated levels of ozone have also been linked to increased visits to the emergency room to treat acute asthma attacks but, interestingly, normal subjects appear to be as susceptible to ozone as asthmatics. However, more studies need to be done, as few to date have compared healthy and asthmatic subjects using the same criteria.
Airborne Allergens
While we know that certain allergens can provoke an acute asthma attack (the average encounter of an asthmatic with a cat will demonstrate this effect), the role of airborne or aeroallergens and chronic asthma can’t be quantified with such assurance. However, there is increasing evidence that chronic asthma and aeroallergens are connected.
Indoor allergens are basic to asthma almost everywhere: house dust mites, mice, cockroaches and pets are common triggers. The usual advice from specialists is to remove — as much as possible — the allergen from the home environment. Studies have demonstrated that this is a very effective method if strictly controlled. For dust mites, all bedding and mattresses should be encased in impermeable covers, bedding should be washed in very hot water (over 130F), and humidity levels kept well in hand.
For cat allergen, removing the animal from a household is the best solution. The allergen is produced by the salivary and sweat glands of the cat (which it then carefully washes all over its coat). Cat hairs spreading throughout the household can keep the allergen in the air for months. But as many people are reluctant to give up a cherished pet, weekly washing of the animal and rigourous cleaning and removal of allergen reservoirs such as carpets can be helpful.
Outdoor allergens depend on geography.
In Europe, olive trees create seasonal allergies the same way that ragweed does in North America. In Australia, researchers compared samples of dust mite and mould spore aeroallergens in both coastal and inland cities. They found that on the damper coast, house dust mites were much more numerous, and increased the relative risk for asthma by 21 times that for the inland city. Other findings from the U.S. also show that there is a striking relationship between elevated levels of house dust mite aeroallergen and increased prevalence of asthma. This adds further credence to the theory that the level of exposure to an allergen is a determining factor in inducing asthma.
Another factor needs to be added to the mix: socioeconomic class. Researchers are increasingly recognizing that a combination of location and social class has an impact on asthma. For instance, inner city children in Atlanta, Georgia, are generally sensitive to cockroach allergens, and not to cats; however, it was precisely the other way around for their suburban counterparts. Moreover, the major allergen for the inner city poor is not house dust mite, as is common in affluent households, but cockroach. Measures to exterminate cockroaches are available, but for this population, moving to an non-infested building may be difficult.
Sometimes, it is social change that can alter asthma statistics. For instance, asthma is not very common in the Third World, but one population in New Guinea saw their asthma prevalence rise from a scant 0.1% in 1960 to 7% by 1990. In those years, the house dust mite count went up dramatically, probably due to the introduction of cloth bedding.
The Interaction Factor
Do environmental pollutants combine with airborne allergens to exacerbate asthma? While the jury is still out on this question, at least one study has shown a relationship between these two factors. Researchers asked asthmatics who were sensitive to seasonal allergies to breathe in 0.12 parts per million of ozone for one hour; they were then tested for any change in airway function. Although the ozone in itself did not seem to have an effect, the sensitivity of the patients to the inhaled allergen increased significantly, showing that a detrimental interaction was indeed at work.
Such studies highlight new areas to be probed, including analyzing the effect on airways of mixtures of pollutants and allergens in different permutations. As the authors point out, the relationship of asthma to the environment is intricate and will be difficult to clarify. However, the more information about this relationship that becomes available, the better equipped clinicians will be to offer therapeutic solutions.
Jennifer is a clinical pharmacologist. She helps doctors of all medical specialties choose medications. Jennifer consults about drug dosage, interactions, and side effects.