Agency for Toxic Substances and Disease Registry (ATSDR) 
Methyl Parathion Expert Panel Report
Issue: Decontamination Criteria

The proposed criteria do not address the decontamination of environmental surfaces that may be sources of exposure. The criteria rely on repeated urine analyses as a basis for determining exposure and, by inference, the potential for future exposure above criteria after urine sampling has terminated. In EPA's removal program, decontamination criteria are used that identify an indoor surface level (using a wipe sample) that "triggers" a decontamination procedure regardless of the urine PNP findings. Such criteria would supposedly add a margin of safety against the failure of urine analyses to predict future exposure and to protect more susceptible potential future occupants. Are such criteria needed and, if so, what would be the basis for establishing quantitative criteria? Also, if decontamination criteria were established, what level of MP should be achieved to declare a residence decontaminated?

The work group recognizes the possibility that biomonitoring of urinary PNP may have some false negative results. The Center for Environmental Health Laboratory that is conducting these analyses reports that, in approximately 1 out of every 200 samples, interfering substances in the urine (as yet unidentified) may prevent detection of urinary PNP. To minimize this possibility, we recommend that in certain circumstances (where there is an imminent hazard of exposure leading to poisoning), relocation criteria should be modified to include consideration of environmental sampling results in the absence of elevated urinary PNP results.

1. In imminent hazard situations, the local public health agency should have the flexibility to advise relocation on the basis of environmental sampling alone, even in the absence of elevated urinary PNP results. An imminent hazard situation is defined as environmental MP contamination that could reasonably result in acute MP poisoning (OP syndrome with cholinesterase inhibition) in a member of the household. A documented case of OP poisoning of household members or pets in an MP-contaminated household should serve as presumptive evidence of an imminent hazard situation.
2. Local public health agencies should also have the flexibility to increase the frequency of urinary PNP biomonitoring if they suspect the possibility of false negative results.
3. Rechecking urinary PNP after members of a household reoccupy a remediated residence would add an additional safeguard to the adequacy of remediation. Whether testing needs to be a continuing practice would be determined by examining the data on postremediation urinary PNPs as they become available.

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