Epidemiological studies carried out since the 1990s have highlighted statistical relationships between indicators of exposure to atmospheric pollutants and the occurrence of excess health events in the population. Toxicological studies have provided explanatory clues to these statistical relationships, by proposing action mechanisms specific to pollutants commonly encountered in the atmosphere. The concordance and coherence of these studies helped to substantiate the causal nature of the effects of air pollutants on health.
Short-term effects are health effects that occur within minutes to weeks after exposure to pollutants. Several hundred works, carried out in many countries, on populations and with different methodologies, have produced a set of convergent results. They show that exposure to pollutants, in particular suspended particles and ozone, is associated with a short-term impact on health. These results were supported by those of large multi-city analyzes such as, in the United States, the NMMAPS study (National Morbidity, Mortality, and Air Pollution Study) and, in Europe, the APHEA study (Air Pollution and Health: A European Approach).
In addition to highlighting the acute effects of urban air pollution, long-term monitoring of cohorts has made it possible to convincingly document the effects of chronic exposure to pollutants, in particular the effect of fine particles on mortality. total and cardiovascular. The results of the first North American studies, published in the early 1990s have since been supported by several European studies.
Exposure to pollutants therefore has not only a short-term effect on the most fragile part of the population; it also has a significant impact on life expectancy. The results of long-term studies show moreover, that these effects are preponderant compared to the acute effects of pollution.
Epidemiological studies show that there are effects of pollution from the lowest concentrations, and that there does not seem to be a protective threshold below which there is no longer observed any health effect. Even in the absence of peaks, air pollution has health effects. Thus, given the relative rarity of extreme pollution events, most of the health impact of air pollution is due to the days of usual pollution.
Through epidemiological studies, it has been shown that characteristics such as age, sex or state of health influence sensitivity to air pollution. Thus certain population groups, such as children, the elderly and individuals suffering from chronic pathologies, have been identified as more concerned by the effects of air pollution.
While the causes of the elderly person’s fragility to air pollution are still poorly understood, certain mechanisms are starting to be considered. Indeed, the elderly would be more sensitive to air pollution due to the decrease in their local antioxidant capacities and the capacity of adaptation of their defense system. In addition, the elderly often present preexisting pathologies such as cardiovascular or respiratory pathologies.
In children, pulmonary maturation is only partial at birth, the stock of alveoli continuing to develop until the age of 8 years. Some studies have shown a link between exposure to pollutants and myocardial infarction through a decrease in peripheral oxygenation, an increase in blood viscosity and changes in heart rate. Thus people with coronary and heart failure are more sensitive to air pollution.
Air pollutants (ozone, VOCs, particles, SO2, NO2) are also aggravating factors for asthma. They increase bronchial reactivity and make individuals more sensitive to allergens. Thus, asthmatics have been identified as more sensitive to air pollution; as well as people with respiratory insufficiency, chronic bronchitis, air pollution favoring decompensations (acute respiratory distress).
Pregnant women also constitute a category of population sensitive to the risks run by the fetus. Indeed, links have been established between exposure to air pollutants and alterations in fetal growth, prematurity and low birth weight. That’s one more reason why early junk disposal services helping to remove pollutants from residential areas would be helpful.
Finally, in some healthy people, the slightest increase in the concentration of pollutants in the atmosphere causes coughing, irritation of the throat or eyes, while others show these symptoms only at much higher levels, or not at all. There is no examination to diagnose hypersensitivity. Only the appearance of evocative symptoms, especially during pollution episodes, makes it possible to suspect it. More research is still needed to better characterize the exposures of these population groups and their responses to air pollution.
A significant number of studies have studied, in vitro or on animal populations (more rarely human), the toxicology of the main pollutants that make up the ambient air. These studies have made it possible to propose mechanisms of action, which provide physiological explanations for the results of epidemiological studies. Although many of these mechanisms are still poorly understood, these studies have provided essential arguments for the biological plausibility of the effects observed in epidemiological studies, and help to substantiate the causal nature of the relationships between exposure to pollutants and health.
Pollution from road traffic poses specific health problems. On the one hand, in direct proximity to traffic lanes, the exposure is greatly increased. Traffic is thus the main source of intra-urban variations in exposure in the majority of urban areas. On the other hand, traffic-related emissions constitute a mixture of specific pollutants (diesel particles, benzene, heavy metals), the levels of which decrease for some quickly (~ 300-500 m) away from the road axis ( ultrafine particles, NOx). While it is still quite difficult today to dissociate the effects of this particular pollution from the effects of more general pollution of the atmosphere, a certain number of studies have made it possible to highlight a specific effect of the pollution emitted by the road traffic on the genesis and occurrence of asthma attacks in children, and mortality and morbidity for cardiovascular causes.
An evaluation carried out within the framework of the Aphekom study relating to 10 European cities and which should soon be extended to Paris, thus estimates that the fact of residing near (within 150 meters) of a major axis circulation (with more than 10,000 vehicles per day) is responsible for 9 to 25% of new cases of asthma in children, and 10 to 35% of cases of chronic obstructive pulmonary disease and coronary heart disease in adults of over 65.
The key message from epidemiology is that the levels of pollutants currently observed are associated with health risks. If the intensity of the effects observed may seem low compared to other risk factors (like tobacco for example), the size of the exposed population is important and therefore the benefit associated with a reduction in the exposure of the population to pollutants would be quite substantial in terms of public health.
As shown for example by the recent results of the Aphekom study, if the levels of fine PM2.5 particles were in line with the WHO quality objectives of 10 µg / m³ on an annual average, the inhabitants of Paris and the surrounding area crown would gain six months of life expectancy.
