A study by Bechtel et al. , 2009, described in the Archives of Environmental & Occupational Health considered polycyclic aromatic hydrocarbons and immune system function in beef cattle. Some cattle were near major oil- and gas-producing areas of western Canada. The mean monthly exposure to PM1. 0 (particulate matter that is <μ 1m in diameter) was approximately μ 7. 1 g/m3 with standard deviation 1. 5. Assume that the monthly exposure is normally distributed