Auto exhaust linked to thickening of arteries

[Guest University of Basel]

Atherosclerosis — or the stiffening and calcification of arteries — is the underlying cause of most cardiovascular disease and related deaths, which are the number one killer in the Western world. A few animal studies conducted in recent years have observed that the inhalation of ambient particulate matter from traffic and other sources accelerates atherosclerosis in rabbits, rats and mice. Very few epidemiological surveys observed more advanced atherosclerosis in areas with higher air pollution, and so far, no study has ever investigated whether the slow but chronic process of the development of atherosclerosis would be affected by ambient air pollutants.

 

In a collaboration of Swiss, Californian, and Spanish researchers, this question has now been addressed using five studies conducted by the Atherosclerosis Research Unit (ARU) of Howard Hodis at Keck School of Medicine University of Southern California (USC). Common to the five double-blind controlled clinical trials was that, in total, each of 1,483 subjects participated during approximately three years in regular measurements - once every 6 months - of the artery wall. The thickness of the artery wall can be measured with ultrasound methods. It is an excellent marker of the degree of atherosclerosis because one of the key features of this very complex and multifactorial pathology is that the artery wall gets increasingly thicker and stiffer as the disease progresses. This process happens over long periods, very silently, and may ultimately – and suddenly – result in a myocardial infarction or stroke.

 

All ultrasound imaging was done by the trained team of Hodis using well standardized methods. Under the lead of Nino Künzli, environmental epidemiologist and vice-director of the Swiss Tropical and Public Health Institute in Basel, Switzerland, the team investigated whether the three-year progression of the artery wall thickness could in part be explained by exposure to ambient air pollution. To do so, novel methods and air pollution models developed by the geoscientist and epidemiologist Michael Jerrett, University of California, Berkeley, have been used to estimate pollution all across Southern California, and to assign to each participant the home outdoor levels of particulate matter, and to estimate the distance of the residence to the next highway or busy roads. Traffic corridors are hot spots of pollution with some pollutants — e.g. ultrafine particles — occurring at 5-10 times higher concentrations then what is observed 100-200 meters away from the roads.

 

Annual progression of the artery wall thickness among those living within 100 meters of a highway was indeed significantly accelerated by 5.5 micrometers/yr or more than twice the average progression observed in these study participants. A positive association between home outdoor fine particle levels and atherosclerosis progression was observed as well, but this finding reached statistical significance only among the socially deprived. Other studies have shown that subjects with faster progression of the artery wall thickness are at increased risk to suffer myocardical infarctions or strokes later in life.

 

While this first-of-its-kind study seems to confirm findings from animal studies, a range of open questions cannot be addressed with this project. In fact, as results were much stronger in subgroups — such as those being treated for high cholesterol, the socially deprived, or those in the treatment groups of these trials — it will be of high interest and public health relevance to understand what the factors are that may determine peoples’ susceptibility to the adverse effects of traffic-related pollution.

 

Larger studies and investigations among less selected groups of the general population are needed to further evaluate this issue. A few studies are currently underway that will indeed target these questions in more detail. Four key studies are being done by the lead institutions of this first study. The USC team is investigating the hypothesis among children and college students. A Spanish team (REGICOR) led by Jaume Marrugat (IMIM Barcelona) is conducting a related investigation by Nino Künzli in the region of Girona, Spain. The Swiss research team, coordinated by Künzli’s group in Basel, just started investigations of the artery wall thickness among the SAPALDIA cohort members recruited 20 years ago.

 

These studies are highly relevant not only because of the dominant role of atherosclerosis in Western societies. The findings add to the emerging evidence that traffic corridors ought to be highlighted as unhealthy residential locations. So far, most air quality regulations ignore these hot spots, although millions of people — particularly in older European towns — live along such street canyons. Policy makers are challenged because these findings raise regulatory questions beyond engine technology, in the areas of urban planning and organization of traffic.