Dermal Absorption of Chlorpyrifos

Geer et al. (2004) provided an interesting comparison between passive dosimetry measures and biomonitoring for chlorpyrifos (CP) exposure in pesticide workers. However, they made a questionable assumption of a 3% dermal absorption factor (DAF) for CP, stating: ‘[The] study by Nolan et al. (1984), supports the use of a 3% DAF, the Environmental Protection Agency (EPA) accepted value, whereas Krieger (1995) supports a 10% DAF and Griffin et al. (1999) report a 1% DAF.’ Thus they appear to treat the differences in these three values of 1, 3 and 10% as if they are caused by experimental error because they failed to recognize that the time of exposure is a determining factor for the CP DAF. Examination of the study by Krieger (1995) shows no experimental data for CP DAF, only a generic range of 0.1–35%/24 h in his Table 2; Nolan et al. (1984) do not show a 3% DAF but a 1.3% average DAF for 12–20 h exposures; Griffin et al. (1995) report an average 1% DAF for an 8 h exposure (Table 1). Apparently the 3% value quoted for Nolan et al. may have come from their abstract where they stated, ‘less than 3% of the dermal dose [of CP] was excreted in the urine’. EPA apparently took that 3% as the conservative upper estimate for DAF in a risk assessment process, but it is wrong to use it in Geer et al.’s study where they are estimating the actual amount of uptake to compare with urinary output. Geer et al. report that in their modeled studies ‘Each worker was monitored once over a period of time representing a standard work shift [8 h] or duration of application . . . Handwash samples were collected over the monitoring period during times when workers would typically wash their hands (i.e. before meals, before smoking, after using the bathroom, and at the end of the monitoring period) to assess surface deposition on the hands.’ Consequently Griffin’s data would lead to an expectation that 1% of the hand loading at the start of the study would enter into the skin after 8 h, and assuming the subjects showered or bathed before going to sleep, some 12–20 h after their body exposure, the expectation from Nolan’s data is that 1.3% would be absorbed into the skin. Consequently, the authors’ usage of 3% for a DAF is very likely to lead to a gross over-prediction of the amount entering the body. Finally, the authors should have divided each subject’s dosage in mg per day by the subject’s body mass to provide their CP dosages in units of mg (kg day) , which could then be compared with EPA’s reference dose (RfD) for CP. Failure to divide by body mass distorts these data because, when reported as mg per day, the surface loadings are multiplied by the subject’s body surface area (BSA)—and that BSA increases with the subject’s weight to the one-half power (Mosteller, 1987). Thus, if they divided their reported mg per day by the subject’s weight, the effect of increasing body mass would be to decrease the mg (kg day) 1 as opposed to increasing the mg per day.