In this study of patients with DVT and healthy controls, residential proximity to major traffic roads was associated with an increased risk of DVT after controlling for individual characteristics and clinical covariates. The increased risk of DVT associated with proximity to major traffic roads was smaller in women than in men and limited to those who were not using oral contraceptives or hormone replacement therapy at the time of diagnosis.
Residential distance from major traffic roads is considered to reflect exposure to traffic emissions,16, 17, 24
as concentrations of particulate matter and other exhaust pollutants have been reported to be higher nearby major roads and then decline exponentially within 500 meters.28, 29
Several previous investigations have shown an association between proximity to major roads and arterial cardiovascular diseases.16-18, 24, 30
In a case-control study in Massachusetts, Tonne and collaborators18
found that living near a major roadway was associated with a 5% increase per kilometer in the risk of acute myocardial infarction. A cohort-study in the Netherlands showed a 1.95-fold increase in cardiopulmonary mortality for subjects living within 100 meters of a freeway or within 50 meters of a major urban road,16
and, more recently, mortality of patients with heart failure has been shown to be 30% greater among subjects from Worcester, Massachusetts living within 100 meters of a major traffic road.17
In an investigation in England and Wales, mortality from stroke was 7% higher in men living within 200 meters of a main road compared to those living =>1000 meters away.30
In our study, the estimates for the increase in DVT risk associated with road proximity were similar in models with or without background PM10
levels as an independent variable, which we estimated using PM10
measurements from ambient monitoring stations that were averaged over broad areas of Lombardia region.9
and road distance were only weakly correlated in our data. This suggests that distance from major traffic roads provided additional information on local spatial variations of air pollution in our study.
A systemic increase in thrombotic tendency, secondary to the induction of inflammatory mediators produced in the lungs and released in the circulation, or to the translocation of particles of smaller diameter from the lungs into the circulation, has been frequently proposed to account for the cardiac and cerebrovascular effects of particulate air pollution6
and may also operate to increase the risk of DVT. The pattern of DVT-road proximity association was remarkably consistent with the results on DVT risks we recently described for background PM10
particularly as both environmental risk factors showed weaker effects in women and no effect in those women who were taking oral contraceptives or hormone replacement therapy.
This study has the advantage of being based on a large number of patients with DVT and controls recruited using a standardized protocol. Patients had objective diagnoses of DVT, and both patients and controls were carefully characterized for inherited and acquired risk factors for DVT. Healthy controls, also due to their selection among non-blood relatives and friends of DVT patients, tended to be distributed in the five study areas with proportions that were very similar to those of the patients. This might have generated overmatching in our study, i.e., distance to major roads of controls might have been more similar to that of patients than they actually are in the population at risk. Therefore, it is possible that the risk of DVT was underestimated in our study. Although the multivariate analysis was adjusted for the major risk factors for DVT, we cannot exclude that other unmeasured confounders might have influenced our results. Living near major traffic roads might have been associated with factors other than exposure to high traffic pollution. Residential location may reflect differences in socioeconomic status. However, no association was found between distance from major road and educational level, a major determinant of socioeconomic status, which was also included in our multivariable models. In addition, one might hypothesize that distance from major traffic roads may be differentially associated with patterns of physical activity. Although we did not have information on physical activity in this study, major differences in physical activity would have likely been reflected in differences in body mass index, which is a strong risk factor for DVT.31
On the contrary, body mass index did not vary in relation to distance from major traffic roads. Also, estimates of DVT risk were adjusted for BMI in multivariable models. Other factors which have been proposed as potential confounders of the association between living near to major traffic roads and arterial cardiovascular disease, such as traffic noise and tobacco smoking, were less of a concern in our analysis, as neither of them is an established risk factor for DVT.31
A recent meta-analysis indicated that diabetic patients might have a moderately increased risk of DVT.32
As diabetes was not known as a risk factor for DVT at the time the present study was initiated, we did not collect information on diabetes that could have been used in our multivariate analyses. Another limitation of the present study is that no information was collected about the floor on which the study subjects lived. Therefore, we were unable to take into account vertical elevation from street level in our analyses.
In our study, the increase in DVT risk associated with proximity to major traffic roads was approximately linear across the range of distances we observed. In previous work, we showed that DVT risk also increased linearly in relation with increasing PM10
concentrations. As particle concentrations have been shown to decrease exponentially with distance from major roads,28, 29
one might have expected a non-linear relationship between DVT risk and distance from major traffic roads. However, some of the previous studies that have associated proximity to major traffic roads to arterial cardiovascular outcomes have also shown no departures from a linear increase in risk,23, 33
or modeled distance from major roads using a linear function.17, 18
The lack of an exponential increase in DVT risk with proximity to major traffic roads may be accounted for by limitations inherent to the use of this surrogate measure as a proxy for traffic exposure. For example, distance of the subjects’ residence from major traffic roads does not take into account individual differences in the time spent at home and in other environments, such as workplaces or in traffic while commuting.22
In our study, we did not have information on low physical activity, a risk factor for DVT that might be associated with distance from major traffic roads and modify the shape of the DVT-traffic road relationship.
In conclusion, our study indicates that subjects living near major traffic roads are at increased risk of DVT. Our findings, particularly if confirmed in future investigations, would demonstrate a common and easy to identify risk factor for DVT, and give further support to the measures aimed at reducing traffic emissions that have been adopted or considered in major metropolitan cities.
Venous thromboembolism is the third most common cardiovascular disease behind acute coronary syndromes and stroke. Exposure to particulate air pollution has been associated with increased risk of arterial cardiovascular disease, but only a few data are available on venous thrombosis. Because road traffic is a major source of exposure to particulate pollution, individuals living closer to major traffic roads might be at higher risk for air pollution effects. In a study of 663 patients with Deep Vein Thrombosis (DVT) of the lower limbs and 859 age-matched controls from Lombardy, Italy, we found that individuals living near major traffic roads had a higher risk of DVT than those living farther away. Using multivariable modeling adjusting for multiple clinical and environmental covariates, we estimated that an individual living AT (within?) 3 meters of distance from a major traffic road had a 47% increase in DVT risk compared to an individual living at a reference distance of 245 meters. The increase in DVT risk was approximately linear over the observed distance range (from 718 to 0 meters), and was not modified after adjusting for background levels of particulate matter measured in the areas of residence of the study participants.
These findings provide novel evidence that living near major traffic roads is associated with increased risk of DVT.