A number of recent toxicity studies have highlighted the increased potency of oxygen analogs (oxons) of
several organophosphorus (OP) pesticides. These findings were a major concern after environmental
oxons were identified in environmental samples from air and surfaces following agricultural spray
applications in California and Washington State. This paper reports on the validity of oxygen analog
measurements in air samples for the OP pesticide, chlorpyrifos. Controlled environmental and laboratory
experiments were used to examine artificial formation of chlorpyrifos-oxon using OSHA Versatile
Sampling (OVS) tubes as recommended by NIOSH method 5600. Additionally, we compared expected
chlorpyrifos-oxon attributable to artificial transformation to observed chlorpyrifos-oxon in field samples
from a 2008 Washington State Department of Health air monitoring study using non-parametric
statistical methods. The amount of artificially transformed oxon was then modeled to determine the
amount of oxon present in the environment. Toxicity equivalency factors (TEFs) for chlorpyrifos-oxon
were used to calculate chlorpyrifos-equivalent air concentrations. The results demonstrate that the
NIOSH-recommended sampling matrix (OVS tubes with XAD-2 resin) was found to artificially transform
up to 30% of chlorpyrifos to chlorpyrifos-oxon, with higher percentages at lower concentrations
(