High concentrations of particulate matter originating from automobile exhausts in polluted air in urban environments can not only lead to respiratory distress, but have also been widely implicated in aggravating atherosclerosis, coronary artery disease, and even inducing heart attacks.

Now researchers from the University of California, Los Angeles (UCLA), the University of California, Irvine, the University of Southern California, and Michigan State University have shown that the smallest particles (<0.18 μm) found in polluted air originating from vehicle exhausts exacerbate coronary problems the most [Araujo et al., Circulation Res. (2007) doi: 10.1161/circresaha.107.164970].

The team exposed mice to ambient air near a Los Angeles freeway for 40 days. One group of mice was exposed to filtered air (FA), another to fine particles (FPs) < 2.5 μm in size, and a third to ultrafine particles (UFPs) < 0.18 μm. The animals exposed to UFPs show the greatest growth of atherosclerotic plaques. The particulate matter also appears to cause a loss of the anti-inflammatory properties of high density lipoproproteins (HDL) in the blood of the mice.

The researchers suspect that the nanoparticles lead to systemic oxidative stress. For example, they measured expression levels of mRNA in the liver from genes that code for antioxidant proteins. The measurements show a 68% increase in expression of the transcription factor Nrf2 in mice exposed to UFPs compared to mice exposed to FPs or FA. Nrf2 is a sensitive gauge indicating the presence of oxidative stress. “We believe that most of the effects relating to both FPs and UFPs are in relation to their ability to stimulate and enhance free radical reactions and that this pro-oxidative potential is greater in the UFPs than in the FPs,” says Jesús A. Araujo of UCLA. UFPs are rich in polycyclic aromatic hydrocarbons (PAHs), compounds that increase the propensity for reactions with oxygen free radicals. “UFPs exhibit twice as much content per mass unit of PAHs in comparison with FPs,” he adds.

However, it is not clear if the increased effects observed for UFPs from vehicle exhausts have any direct bearing on the potential effects of exposure to engineered nanoparticles