(We assume that the use of a greater than 100% reduction in combined pollutants signifies that individual percentage falls for each of the four pollutants was recorded, and these values added together by the authors.)
However, self-styled 'environmental' groups have tried to implicate vehicle exhaust emissions as an explanation for the apparent dramatic rise in asthma cases in developed countries in recent decades.
“Air pollution does not cause asthma.”These are not quotes from the Association of British Drivers. They are taken from the 1995 Department of Health Study on the Causes of Asthma, by Dr Kenneth Calman, Government Chief Medical Officer.
“There is no correlation between levels of vehicle emissions and asthma incidence.”
"...the available evidence does not support a causative role for outdoor air pollution"and
"Factors other than air pollution are influential with regard to the initiation and provocation of asthma and are much more important than air pollution in both respects."
"In the last 40 years the level of (outdoor) air pollution has decreased dramatically"
"Nevertheless the incidence of asthma has risen"
"Pollution does not cause bronchitis or asthma, nor does wearing a so-called anti-pollution mask do anything except identify the wearer as a sucker"
"One reason is our over-hygienic environment ... fighting infection makes the body produce less of the gene that brings on allergies ... It is clear our sterile, Westernised living style has something to do with it."
As air quality has improved, so the number of asthma cases has risen. Can cleaner air cause more asthma?
Apart from an element of this increase due to diagnosis (see below), the increase in asthma cases is attributed by groups such as the British Allergy Foundation to factors inside the home, not on the roads.
Dr Martin Stern of the British Allergy Foundation has categorically rejected that asthma is linked to outdoor air pollution. Instead, he links it to the household dust mite and its excreta. Modern living, with central heating, draught-free doubleglazing, fitted carpets and poor diet plus lack of exercise, provide the ideal environment for the dust mite and its effects on our respiratory system. The proportion of homes with fitted carpets in England, France and Italy is directly proportional to the incidence of asthma in each country.
Scientists at Nottingham University have found that a woman taking antibiotics during pregnancy increases the chance that the baby will get asthma by 43%, most likely as a result of impaired development of the unborn child's immune system. Dietary factors may be the single biggest cause of the increase in asthma incidence, according to Professor Anthony Seaton of Aberdeen University, where a team of researchers report that the wrong diet, particularly a low consumption of fresh fruit and vegetables, can lead to an eightfold increase in the risk of getting asthma. Perhaps the most useful breakthrough comes fromm detailed research into the mechanism by which house dust mites are implicated not just in the provocation of asthma, but in initiating (causing) it (National Asthma Campaign). The NAC says that house-dust mite droppings are the number one asthma trigger, but also points out new evidence that the mites can actually cause asthma to develop in the first place. This important discovery was made by a team of researchers at St George's Hospital Medical School, London in 1999. It was the first research showing that enzymes in house-dust mite excreta destroy the biological 'glue' that holds together the protective cells in human airways. Once this protective lining has been breached the enzymes are free to interact with the same critical system that antibiotics may impair -the body's immune system — leading to asthma.
Heavily polluted eastern European countries have very low incidences of asthma. Conversely, New Zealand, which is noted for its clean air, has amongst the highest incidence in the world per head of population. The idyllic ocean island of Tristan da Cunha has some of the cleanest air on the surface of the planet, yet medics there report that virtually every inhabitant has asthma.
Dr Jeff Llewellyn, of the Government Buildings Research Establishment, has shown that the air in the average UK home is 10 times more polluted than city smog. Asthma symptoms aggravated by outdoor pollution are linked to sulphur dioxide, nitrogen dioxide and particulate pollution (PM10s). These are pollutants which, in terms of traffic, are linked to diesel bus fumes more than car exhausts. The National Environment Technology Centre has shown that a single diesel engined bus emits 128 times as much particulate pollution, and 39 times as much oxides of nitrogen, as a single petrol engined car.
Read on for more details.
The immune system in children of that age is still developing, and if it encounters the allergens does not react violently to them, but is 'inoculated' — the proper term is 'sensitised' — against them. If, decades later, sensitised adults again encounter allergens, then their immune system does not react violently. However, if adults encounter allergens not experienced in infancy, then the immune system reacts violently (is 'provoked'). Thus, the situation is analogous to infectious diseases -adults who have never encountered them have much worse reactions to the common childhood diseases (e.g. measles, mumps, chickenpox) than do children.
The growth of mites, fungi and moulds is supported by the warm and moist conditions in modern homes — due to their central heating, and dramatic reduction of ventilation (in order to reduce heat loss). It is the waste from house dust mites that is considered to be the single greatest cause of the increase in asthma in developed countries.
The above combustion products are, of course, generated by trains, buses, planes, power stations, domestic fuels and tobacco smoke, as well as cars. For all sources, except trains, buses, cooking gas and tobacco smoke, the combustion products are emitted into open air where their concentration is too low to be of any health significance. However, trains and buses often operate from, and sit with idling engines in, enclosed stations, where the combustion products (as also the concentration of diesel particulates) reach very much higher concentrations than on open streets. To the extent that combustion products can cause asthma (and cancer), passengers, and particularly staff, frequenting enclosed rail and bus stations would be more at risk of asthma than those who do not.
Potentially harmful combustion emissions from cars have fallen dramatically in the past several decades due to better control of combustion and, more importantly, the use of catalysts. Trains, buses and planes have been allowed to escape having catalysts fitted to them (another subsidy) — a bus now emits about a hundred times more potentially asthma inducing pollutants than a car. Cars are not a cause of asthma.
The highest concentrations of combustion products from fuel occur in kitchens where gas is used for cooking — the combustion products immediately pass into the air of the kitchen. As noted in the discussion of allergens, kitchens in developed countries have become poorly ventilated, and the concentrations of carbon monoxide and nitrogen oxides within them are by far the highest that most people experience.
Other gases are emitted in homes by, for example, glued articles (particularly, formaldehyde from fibreboard), man made fibres in carpets, dry-cleaned clothes and air fresheners; these gases are organic (i.e. contain carbon), and have the potential for causing asthma (and cancer).
Research by the U.S. Environmental Protection Agency shows that air pollution within the modern home can be up to 70 times (7000%) higher than outside, because of lack of ventilation.
"Childhood asthma is a greater problem in the green fields of East Anglia than in London" (Professor Stanley Feldman).
It is the same in the Western Isles of Scotland as in London, it shows no correlation with the distribution of combustion products — whether from power stations, trains, buses, planes, or cars. Allergenic asthma is low in areas of poor living conditions — where infants are exposed to dirt.