Positive Health Online

On a cold windless December eve in 1991, a friend of mine with mild asthma arrived home from a nightmare bus journey through London. Her chest felt tight. She was short of breath and was wheezing painfully. Unusually for her, she had to use her inhaler. Too late, she heard on the news that hourly nitrogen dioxide levels had peaked at 423 ppb (WHO health guidelines are 104ppb) and that people with pre-existing heart and lung problems were advised to stay indoors. She had no doubt that air pollution had led to her asthma attack.

There is no doubt that air pollution contributes to asthma and other respiratory problems but contrary to popular belief, there is doubt that air pollution directly causes asthma. Other environmental factors together with genetic factors are likely to be even more important.

Certainly, air pollution can be lethal. In December 1952, the people of London experienced a killer smog, when fog, black smoke and sulphur dioxide were trapped beneath a still cold air mass. Over a period of one week, there were 4,000 more deaths than expected.

The deaths were due to heart and respiratory problems. The pollutants were all from coal burning. Since then, coal fires have been replaced by central heating and industry has moved away from urban areas. As a result, air quality in British cities improved substantially.

But the threat to health from air pollution did not end with the demise of coal. In December 1991, pollutants and fog again accumulated beneath a still cold air mass in London. Again, more people died than were expected to, though not on the same scale as in 1952. It is estimated that roughly 160 deaths were due to the 1991 smog. There is no record of the degree of suffering from non-fatal asthma attacks. This time, the culprit was not coal burning but the pollutants from more than two million motor vehicles clogging London's roads.

A number of pollutants are emitted from traffic but the main one in this smog was nitrogen dioxide.

Clinical studies have demonstrated that high levels of sulphur dioxide and nitrogen dioxide make asthma symptoms worse. These levels can occur in Britain during severe winter smogs. Ozone is a summer pollutant, formed on hot sunny still days, when sunlight triggers chemical reactions between the nitrogen oxides, volatile organic compounds and carbon monoxide emitted from motor vehicles. At high levels, ozone can cause short-term breathing difficulties in healthy people and can also make attacks worse for people with asthma.

These pollutants may increase susceptibility to respiratory tract infections. Particulates are the tiny particles of carbon in black smoke. In London, where there are many diesel vehicles, no heavy industry, and few power stations, 96% of the particulates in the air come from road traffic. Particulates may be carrying carcinogenic chemicals such as PAHs (polycyclic aromatic hydrocarbons). The smallest particulates can penetrate deep into the lungs, past the defense mechanisms of the respiratory tract.

The current consensus is that air pollutants can make existing health problems worse but are not thought to be the cause of these health problems. Asthma is a chronic and potentially serious lung disease affecting some three million people in Britain and causing nearly 2,000 deaths each year. It is caused by a combination of environmental factors such as exposure to allergens and genetic susceptibility. It is aggravated by other environmental factors such as exposure to cold air, tobacco smoke, viruses, and indoor and outdoor air pollutants.

From 30 to 50% of the population is atopic, meaning they have a genetic tendency to develop allergies. When atopic babies are frequently exposed to an allergen, they become sensitised to that allergen. Further contact with that same allergen, even many years later, can lead to allergies such as asthma, eczema and rhinitis. About 90% of children with asthma are atopic as are half the adults.

In Britain, the most common allergens are not pollutants but natural substances such as proteins from the house dustmite, pollen and pet dander. Except for pollen, the main threat to asthma sufferers is from the indoor environment. The most common allergen is Der pI – a protein in the faeces of the microscopic dustmite. Most homes have hundreds of thousands of dustmites, living in mattresses, carpets and upholstered furniture. They feed on dead skin cells which humans shed at the rate of half to one gramme a day. 500 dustmites are all that is needed to produce enough Der pI to set off an acute asthma attack in a susceptible person.

My friend went to her GP and discovered that she was indeed allergic to Der pI. Daily exposure to Der pI at home was maintaining her sensitivity to other environmental triggers such as nitrogen dioxide. It is not realistic to eliminate the dustmites completely but there are control measures that can be taken to avoid Der pI. The most effective tactics are to cover mattresses, duvets and pillows with special covers impermeable to Der pI and to remove dust-collecting furniture and carpets. The Der pI is readily removed by damp-dusting from materials such as vinyl, wood, leather, and tiles. There's not enough evidence to support claims that ionisers, high efficiency particulate filters or expensive vacuum cleaners improve the symptoms of people with dustmite allergy. But all these measures require a change in lifestyle and considerable expense, neither of which my friend was prepared to make.

Controlling air pollution by reducing traffic would go a long way towards reducing asthma suffering but must also be accompanied by measures to reduce exposure to allergens at home if asthma is to be effectively prevented.