Several epidemiologic studies1–6 have shown an association between occupational exposure to fossil fuel ash and respiratory health effects. The early studies1–2 reported clinical symptoms and signs, such as upper respiratory tract irritation, a cough with dyspnea, and rhonchi on physical examination resulting from occupational exposure to fuel oil ash. This suggests that fuel oil ash is a respiratory irritant capable of inducing respiratory symptoms, including those suggestive of asthma. However, these early studies1–2 had limited measures of exposure and effect, and the exposures were generally extremely high compared to current occupational exposure. More recently, a small study3– of 17 men cleaning bottom ash from an oil-fired boiler found marked reductions in lung function (FEV1). In another study of respiratory symptoms, Levy and Hoffman4 investigated a large outbreak of “boilermakers’ bronchitis,” described as asthmatic bronchitis, after 100 boilermakers worked on the oil-to-coal conversion of a utility company power plant. Studies5–6 on acute (ie, across a work shift) and short-term (over several weeks) changes in FEV1 demonstrated decrements in lung function among workers who overhauled an oil-fired boiler.