Researchers determined the potential associations between exposure to airborne fine particulate matter (ie, particulate matter that is < 1 μm in diameter, or PM1.0) and polycyclic aromatic hydrocarbons (PAHs) and immune system function in beef cattle by using blood samples collected from yearling cattle in 22 herds in the spring of 2002. The herds were located at variable distances from industry field facilities in the major oil- and gas-producing areas of western Canada. The researchers evaluated immune system competence by measuring populations of B-lymphocytes and T-lymphocyte subtypes (CD4, CD8, γδ, and WC1) in peripheral circulation (n = 469), and systemic antibody production in response to vaccine administration (n = 446). They used particulate air monitors to estimate the exposure of the cattle to airborne contaminants by determining mean monthly concentrations of PM1.0 and 24 different PAHs from January to June. The mean concentration of PAHs measured in the ambient air of herds monitored in this study was low, with naphthalene being present in the highest concentration (geometric mean = 5.6 ng/m3; geometric standard deviation = 38), followed by 1-methylnaphthalene (geometric mean = 2.2 ng/m3; geometric standard deviation = 12). The geometric mean monthly exposure to PM1.0 was 7.1 μg/m3 (geometric standard deviation = 1.5) for the same period. The researchers detected no significant plausible associations between exposure to any measured airborne contaminants and immune system function.