Évaluation de l’exposition à la pollution atmosphérique liée au trafic routier dans les études épidémiologiques : une revue de littérature

Heck J, Malagoli C, Del Giovane C, Vinceti M. A review and meta-analysis of outdoor air pollution and risk of childhood leukemia

Leukemia is the most frequent malignant disease affecting children. To date, the etiology of childhood leukemia remains largely unknown. Few risk factors (genetic susceptibility, infections, ionizing radiation, etc.) have been clearly identified, but they appear to explain only a small proportion of cases. Considerably more uncertain is the role of other environmental risk factors, such as indoor and outdoor air pollution. We sought to summarize and quantify the association between traffic-related air pollution and risk of childhood leukemia, and further examined results according to method of exposure assessment, study quality, leukemia subtype, time period, and continent where studies took place. After a literature search yielded 6 ecologic and 20 case-control studies, we scored the studies based on the Newcastle-Ottawa Scale. The studies assessed residential exposure to pollutants from motorized traffic by computing traffic density in the neighboring roads or vicinity to petrol stations, or by using measured or modeled nitrogen dioxide and benzene outdoor air levels. Because heterogeneity across studies was observed, random-effects summary odds ratios (OR) and 95% confidence intervals (CI) were reported. Whenever possible we additionally conducted stratified analyses comparing acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Limiting the analysis to high-quality studies (Newcastle-Ottawa Scale ≥ 7), those using traffic density as the exposure assessment metric showed an increase in childhood leukemia risk in the highest exposure category (OR = 1.07, 95% CI 0.93-1.24). However, we observed evidence of publication bias. Results for NO2 exposure and benzene showed an OR of 1.21 (95% CI 0.97-1.52) and 1.64 (95% CI 0.91-2.95) respectively. When stratifying by leukemia type, the results based upon NO2 were 1.21 (95% CI 1.04-1.41) for ALL and 1.06 (95% CI 0.51-2.21) for AML; based upon benzene were 1.09 (95% CI 0.67-1.77) for ALL and 2.28 (95% CI 1.09-4.75) for AML. Estimates were generally higher for exposures in the postnatal period compared to the prenatal period, and for European studies compared to North American studies. Overall, our results support a link between ambient exposure to traffic pollution and childhood leukemia risk, particularly due to benzene.

[Health risk assessment of traffic-related air pollution near busy roads]

BACKGROUND

Although ambient urban air pollution has well-established health effects, epidemiology faces many difficulties in estimating the risks due to exposure to traffic pollutants near busy roads. This review aims to summarize how exposure to traffic-related air pollution near busy roads is assessed in epidemiological studies and main findings regarding health effects.

METHOD

After presenting the specificity of emissions due to traffic road, this review identifies the key methods and main results found in epidemiologic studies seeking to measure the influence of exposure to nearby traffic on health published over the past decade.

RESULTS

The characterization and measurement of population exposure to traffic pollution faces many difficulties. Thus, epidemiological studies have used two broad categories of surrogates to assess exposure: direct measures of traffic itself such as distance of the residence to the nearest road and traffic volume and modeled concentrations of pollutant surrogates. Studies that implemented these methods showed that people living near heavy traffic road or exposed to near-road air pollution tend to report more health outcomes.

DISCUSSION

Traffic-related air pollution near busy roads is the subject of increasing attention, and tends to be better characterized. However, its health impacts remain difficult to grasp, especially because of the vast diversity of approaches used in epidemiological studies. Greater consistency in the protocols would be desirable to provide better understanding of the health issue of traffic in urban areas and thus to better implement policies to protect those most at risk.

[Allergic respiratory diseases and outdoor air pollution]

INTRODUCTION

After having increased for some time, the prevalence of allergic diseases may have reached a plateau. During this increase, considerable concomitant changes in air pollution have occurred. Photo-oxidant air pollution, related to traffic, has become preponderant. The implication of air pollution in the epidemic of allergies is still debated.

BACKGROUND

Experimental studies have suggested that the effect of air pollutants, including particulates and ozone, on the worsening and even the induction of allergies is biologically plausible. In addition, epidemiological studies have shown a short term impact of the peaks of air pollution on exacerbations of asthma. On the other hand, the results of epidemiological studies dealing with the long-term effects of chronic exposure to air pollution on the prevalence of allergies are less consistent.

VIEWPOINTS

The implementation of new-born cohorts, the use of dispersion models to improve exposure assessment and the study of gene-environment correlations, should increase our knowledge of the role of traffic-related air pollutants in the development of allergies and identify subjects more sensitive to their effects.

CONCLUSIONS

Some traffic-related air pollutants may have played a more important role in the increase in the prevalence of allergies than was assumed from the first epidemiological studies.

Retrospective assessment of exposure to traffic air pollution using the ExTra index in the VESTA French epidemiological study

This study applies a traffic exhaust air dispersion model (the ExTra index) to 403 children enrolled in a French multicentric case-control study, the VESTA study (Five [V] Epidemiological Studies on Transport and Asthma). The ExTra index (previously validated by our team) was used to assess lifelong average traffic-related air pollutant (TAP) concentrations (nitrogen oxides) children in the study were exposed to in front of their living places. ExTra index took into account traffic density, topographical parameters (building height, road and pavement width), weather conditions (wind direction and strength) and background pollution levels. Topographical and traffic data were collected, using a specific questionnaire for each home, school or nursery address, attended by children. The assessment of time-weighted NOx levels in front of the children's living places highlighted significant disparities: mean ExTra index values and share attributable to proximity traffic were, respectively, 70+/-42 and 14+/-22 microg/m3 NOx equivalent NO2 for the 403 children in our study. Not only would this diversity not have been revealed using urban background pollution data provided by air quality networks, it would have resulted in 40% of the children being misclassified with regard to their TAP exposure by underestimating it in half of the cases and overestimating it in the other half. Such errors of classification, which are highly prejudicial in epidemiology, argue strongly for the use of an index such as the ExTra, which enables TAP exposure to be reconstructed within the framework of retrospective or prospective epidemiological studies.