Health Effects of Gaseous Air Pollutants

JON G. AYRES

1 Introduction

Gaseous air pollutants constitute an important overall component of both outdoor and indoor air and are recognized to cause health effects, essentially in individuals with pre-existing disease. For the purposes of this chapter the gases, the primary pollutants sulfur dioxide (SO2), nitrogen dioxide (No2) and carbon monoxide (CO) with the secondary pollutant ozone, will be considered whereas acidic species will not as they are generally regarded as part of the particulate fraction. As it is likely to be the acidic nature of those species that are important in health terms, even those acids which are present in the air as a vapour phase will not be considered here.

The sources of these pollutants are important when considering health effects because sources relate to individual and population exposures. The main source of SO2 is from fossil fuel burning, the major contributors in the UK being coal-fired power stations. Nitrogen dioxide is derived from vehicle emissions, industrial sources (including power stations) and, in the indoor environment, from combustion of gas. Although for smokers of cigarettes the major contribution to their CO exposure by far comes from their habit, in ambient air the main source is again traffic derived. Ozone is formed by the action of ultraviolet light on oxides of nitrogen and hydrocarbons, so is essentially a pollutant of the summer months in climates such as the UK but may be more perennial in countries where sunlight is present all year round. Ozone levels are generally higher downwind from a city because of the atmospheric chemistry of the formation of ozone combined with the fact that ozone, a very reactive gas, is quickly neutralized by nitric oxide in urban areas.

The health effects of gaseous pollutants have been determined in a number of different ways:

1. By chamber (human challenge) studies

2. By studies of morbidity (e.g. symptoms, inhaler use), usually in panels of subjects perceived to be at risk

3. From studies of hospital admissions (i.e. routinely collected data)

4. From studies of mortality

Chamber studies enable the effects of individual pollutants to be studied alone or in combination with other pollutants on volunteers under strictly controlled conditions. The chief ad vantage of this type of study is that accurate doses can be delivered and the effects of selected co-factors assessed. However, the volunteers involved in such studies are usually normal subjects or patients with mild asthma who tend to be younger, in contrast to the older subjects who are more likely to be affected by air pollution. Additionally, in chamber studies, the duration of exposure is relatively short compared to outdoor, real-life exposures and consequently it may be difficult to extrapolate findings from these types of studies to the effects that would be seen in the overall population exposed to the outdoor environment. Children are not studied in these types of experiments for ethical reasons, which prevents study of an age group where asthma is very common and in whom the health effects of pollution are often perceived to be significant. However, despite these caveats, chamber studies have, in general, provided very useful information as to the presence or absence of effects of specific pollutants at specific doses and have provided useful insights into the mechanisms of these effects.

Epidemiological studies have been much more informative about health effects both at an individual and population level, studying as they do the real-life situation. The difficulty comes in deciding how large an effect may be and to what specific pollutant or pollutant mix such an effect may be attributable. On a day-to-day basis, exposure to air pollutants may have an immediate effect, either on the same day as a rise in air pollution or perhaps delayed, lagging two, three or more days after a rise. In some situations the cumulative or average exposure over a period of three days or more may be important in determining health outcome. It is even possible that longer lags may be more important for differing health end points, an area which is currently being explored.

There is no doubt that there is a range of sensitivities to pollutants across different 'at risk' groups in terms of health effects of air pollution. Patients with pre-existing lung and heart disease appear to be particularly at risk, notably patients with asthma and chronic obstructive pulmonary disease (COPD). More recently, the effects of particulate pollution on patients with coronary heart disease and cerebrovascular disease have been identified, but the role of gaseous pollutants in these two disease categories is not so clear. Asthma is a common condition, affecting around 6% of the total population of the UK. In this condition, the lining of the bronchial tree is inflamed and unduly sensitive to external triggers, such as allergens in those sensitized, viral infections or physical stimuli such as exercise or inhaling cold air. Consequently, these patients a re not only important as a risk group for the effects of air pollution but also act as a group where changes in lung function are frequent and measurable when trying to define the presence and size of an effect from an external stimulus. COPD is essentially a disease of cigarette smokers and although, like asthma, it is also an inflammatory condition, on a day-to-day basis these patients show no marked changes in lung function. Patients with either COPD or asthma develop symptoms because of the airway narrowing resulting from the inflammatory process. Where the baseline airway diameter is small, only minor reductions in diameter can produce marked reductions in airflow and hence symptoms. However, it is at least intuitively logical that, for respiratory diseases, inhalation of polluted air can lead to a deterioration in symptoms.

Coronary heart disease and cerebrovascular disease share a common pathogenesis characterized by the formation of atheroma in the arteries supplying the heart or brain, respectively. In contrast to diseases of the respiratory tract, it is not entirely clear at present how inhalation of air pollutants can lead to vascular health effects, but associations have been shown between ischaemic heart disease deaths and ozone, although the major impacts in this disease area appear to derive from particulate exposure, so further discussion falls outside the remit of this chapter.

It is important to recognize that there may well be interactions between different elements of the pollutant mix in determining health effects. The statistical analysis of time series data (i.e. following individuals over long periods of time or considering hospital admissions and mortality over periods of time) will...