Issue: Environmental Fate and Biomonitoring
What are the appropriate environmental sampling methodologies to characterize the extent of contamination in indoor settings?
The panel was informed that applications of methyl parathion in residences were made by numerous methods such as broadcast and spot application. The frequency and amount applied in these cases was not systematic and varied widely among residences. It is the opinion of the Risk Identification/Management Work Group that broadcast applications have the greatest potential for adverse human health effects.
The principal route of exposure route is most likely dermal, especially in infants and young children. In the latter, there may also be smaller amounts of exposure through the respiratory route and an undetermined amount by the oral route.
The relative amounts of environmental degradation products of MP in these settings are not clearly known and may be important because of formation of small amounts of para-nitrophenol (PNP). If MP is resulting in significant PNP production, then a mixture of MP/PNP may be exposing individuals. Dr. Riviere’s laboratory has shown that when PNP and ethyl parathion are co-dosed, two events may happen:
- PNP enhances ethyl parathion absorption, and
- PNP is rapidly absorbed and shows up in urine as PNP.
No similar studies have been done with MP. The importance of this data is that “old” MP (i.e., MP that has been in houses for prolonged periods) may have PNP, which would be reflected in increased PNP in urine test results. Such results do not reflect MP exposure and would indicate minimal toxicological risk.
The effects of PNP on MP have not been studied. Similarly, Dr. Riviere’s group has shown that exposure of parathion simultaneously with other pesticides may modulate their absorption, often blocking it (e.g., as fenvalerate does). Therefore, these effects with MP may be important and should be investigated in a relevant animal model for human absorption such as the pig or monkey.
- The topic of environmental degradation products of methyl parathion in these settings is a critical data gap. We recommend that the agencies monitor surfaces for MP and PNP in representative samples of homes to determine the need for systematic sampling for these degradation products in all homes.
- The work group agrees with the steering committee that baseboard wipe sampling is acceptable as a general index of household contamination because it appears that MP was applied to these surfaces in all cases. Baseboard sampling should be mandatory in all suspect residences. A systematic sampling protocol for baseboards that specifies which locations, as a minimum, are to be sampled appears reasonable. The work group cautions that, although baseboard sampling is a reasonable screen for the presence of contamination, these samples may not accurately reflect the true potential and degree of exposure and must be supplemented with additional sampling as described below.
- The work group concurs with the steering committee’s assessment of the need for additional mandatory sampling, the type and location of which is determined at the time of on-site inspection. The work group supports the use of a household environmental exposure questionnaire to assist in determining the most appropriate locations for additional sampling. These additional samples should include surfaces with dislodgeable residues where contamination is suspected and exposure is most likely.
- Air samples for MP should be performed in rooms with known or suspected contamination of heating systems. Air samples should be taken with the heating systems on and at sampling heights that reflect exposure to infants and toddlers.
- The work group recognizes that there may be important regional differences in environmental factors (such as climate and type of residences) that affect household MP exposure. If the discretionary sampling identifies important regional differences, regional agencies should have the flexibility to adjust their environmental sampling protocols to minimize misclassification of contaminated homes.
What are the health endpoints of concern associated with chronic exposure to methyl parathion in indoor settings?
Methyl parathion is an organophosphorous (OP) insecticide of the phosphorothioate group. According to conventional wisdom, OP insecticides or their active metabolites elicit toxicity by inhibition of nervous system acetylcholinesterase (AChE). Inhibition of AChE leads to accumulation of the neurotransmitter acetylcholine (ACh), leading to hyperactivity in cholinergic pathways present in the central nervous system and peripheral nervous system, and both automatic and somatic pathways within the peripheral nervous system. Thus the resultant hypercholinergic activity leads to a variety of signs and symptoms, some of which (the respiratory) can be life-threatening if poisoning is from a sufficiently high level.
MP requires metabolic activation to methyl paraoxon (MPO) to yield appreciable anticholinesterase activity; MPO could phosphorylate serine esterases other than AChE or serine proteases. Inhibition of these other enzymes, if they are noncritical enzymes, could be protective (they could have a scavenger function) or, conversely, could yield toxicities unrelated to AChE inhibition.
It is the opinion of the work group that peripheral neuropathy, which has been seen with other OPs, is not a consequence of MP exposure. Also, evidence is not great for other target organ toxicities at doses lower than those causing neurotoxicity.
Blood cholinesterase (ChE) inhibition is a biomarker of exposure. However, agreement does not exist regarding how much blood ChE inhibition correlates with nervous system AChE inhibition or how much AChE depression is required for neurobehavioral toxicity. Several detoxication enzymes exist that can potentially degrade MP or MPO. If these detoxication reactions occur efficiently enough, then MPO will not have the opportunity to inhibit ChE, either in the brain or the blood. It is expected that effective detoxication will occur at low-dose levels. Thus, if analytical chemistry techniques are sensitive enough, urinary PNP will reflect lower exposure levels than red blood cell (RBC) ChE inhibition will. The body is well adapted to respond to change and stressors (chemical or otherwise). Maintenance of homeostasis would be expected whenever any AChE inhibition leads to hypercholinergic activity through down-regulation of neurochemical functions. The magnitude of such homeostatic compensation at different ages and in different physiologic states is not known. For this reason, biomarkers of exposure may not be entirely predictive of biomarkers of effect.
Neurobehavioral effects have been noted experimentally following MP exposure in experimental animals; it is logical that this would be one of the major, if not the major, potential toxicity against which public health policy needs to protect.
Infants and children have immature low levels of xenobiotic metabolizing enzymes and lower renal clearance rates. This lower level of protection (compared to that of adults) makes them more sensitive to the effects of MP because they achieve higher internal doses. Because both AChE and acetylcholine appear to be morphogens within the developing nervous system, perturbing the structure or function of AChE and elevation of ACh levels could have deleterious effects on the development of normal connections in the maturing nervous system.
Much of the above is logical extrapolation to humans from data generated in experimental animals. One critical data gap is the sensitivity of human blood, liver, and brain esterases to inhibition by MPO and the likelihood of MPO degradation by plasma and liver A-esterases; the biochemical protection available to humans is not known and, therefore, predictions of the disposition and internal dose of MPO cannot be made. A second critical data gap is the development of these same enzymes in humans; the vulnerability of the infant or child cannot be predicted compared with that of the adult.
Exposure to MP can result in a range of signs and symptoms that are dose- and host-dependent. These symptoms can range from subtle neurobehavioral disturbances to nonspecific symptoms such as nausea; diarrhea; dizziness; confusion; blurred vision; excessive sweating, tearing, and drooling; weakness or muscle twitching; to acute cholinergic crisis with severe manifestations of the above symptoms. Direct experience in locations where indoor spraying has occurred indicates that most household members are likely to be asymptomatic or have low-grade symptoms.
What populations are susceptible to adverse health effects as a result of exposure to methyl parathion?
The work group agrees with the steering committee that there are high-risk groups in the general population who are likely to be more susceptible to developing toxicity from MP exposure. These high-risk groups require greater levels of protection, which would include more intensive biomonitoring for exposure and lower thresholds for public health interventions. The work group also agrees with the steering committee that age is an important risk factor for MP toxicity. In addition, the work group has identified the following high-risk groups not considered by the steering committee:
- The fetus (and by extension, pregnant women). The developing nervous system is a potential target organ for MP neurotoxicity (see discussion, pp. 4-5). The degree of susceptibility and the degree of protectiveness afforded by maternal clearance and defense mechanisms compared with those of infants are unknown.
- Infants and children. Immaturity causes low levels of xenobiotic metabolizing enzymes and decreased renal clearance, making infants and children more susceptible than are adults to MP toxicity. In addition, the brain continues to develop after birth and continues to form neural connections through adolescence (age 16). Younger children are likely to be more susceptible than older children, but the magnitude of risk at specific age groups is unknown.
- People who have impaired hepatic or renal clearance (e.g., elderly with poor liver or kidney function or people with liver or kidney diseases that impair clearance).
- People using medication for chronic diseases, or other purposes, whose pharmacokinetics may be affected by MP exposure (e.g., people with kidney or liver transplant).
- Mentally disabled people who may not be able to adequately comprehend and evaluate risk or follow public health interventions based primarily on health education.
- People who have other sources of exposure to OPs (e.g., occupationally exposed people).
- The work group agrees with the steering committee proposal to have risk-group-specific urinary PNP relocation action levels.
- Infants (children less than 12 months of age) are clearly a very high risk group and require the lowest urinary PNP relocation action levels.
- It is the opinion of the work group that the fetus may be as susceptible to MP neurotoxicity as infants, or more so. The degree of protection afforded by the mother is unknown. The steering committee should weigh these considerations and review any available scientific evidence that addresses fetal susceptibility and maternal protection. If the data suggest that maternal protection does not afford ample protection to the fetus, or if there are insufficient data to resolve this issue, then putting pregnant women into the highest risk group, along with infants, would improve the public health protectiveness of the relocation criteria.
- Because the central nervous system continues to develop and mature through adolescence, the age band for children should be expanded through adolescence (age 16). The steering committee’s proposed cutoff of 5 to 6 years was based, in part, on age-specific differences in opportunities for exposure. Because urinary PNP is a biomarker of exposure, an elevated urinary PNP indicates exposure, regardless of the underlying behaviors leading to that exposure.
- Although separate action levels based on urinary PNP levels for pregnant women and children are reasonable, individual risks among members of other high-risk groups are likely to be highly variable and situation-specific. We recommend that local public health officials have the flexibility to exercise judgement in evaluating biomonitoring results from members of other high-risk groups and be allowed to recommend public health interventions accordingly.
Given the specific characteristics of indoor exposure to MP, how do we monitor a population to adequately evaluate exposure?
There is a preponderance of evidence that monitoring urinary metabolites of OPs is a good index of low-level exposure. Studies in humans have shown that urinary measurement of PNP is a good index of MP exposure. The Morgan study of human volunteers demonstrated substantial excretion of this metabolite within 24 hours after oral exposure. Many limitations exist in the Morgan study including oral vs dermal route, small sample size, incomplete measurements of PNP and dialkyl phosphates, and no mass balance. In addition, the elimination kinetics measured in this intermittent oral dosing study may not accurately reflect steady state conditions or the intermittent dermal dosing likely to be occurring in the field. Sustained dermal absorption should produce more consistent urine PNP data. It is also possible that absorption is occurring from multiple routes. For these reasons, longer urine collections are required.
Although a 2-hour urine sample would be preferable, field experience has demonstrated that this procedure is not practical in this situation. In previous biomonitoring conducted by EPA and ATSDR, random samples have been extrapolated to 24-hour urine volume. A preferred method would be creatinine adjustment by age, sex, and weight. It is critical to learn what the variation in urinary PNP excretion will be under the proposed sampling protocols to evaluate the protocol’s ability to reasonably estimate MP dosing under various exposure scenarios. Until these data become available, the work group cannot assess how closely spot collections of urinary PNP estimate actual MP exposure.
In addition to urine sampling, an individual exposure questionnaire should be administered before urinary PNP sample collection to ensure that the sample has a reasonable likelihood of assessing exposure.
- The work group has identified the variability in spot samples of urinary PNP (both day-to-day and diurnal) as a critical data gap. Until this variability is measured, the ability of the proposed biomonitoring protocol to estimate MP dosing under various exposure conditions is undetermined. The pilot data from Memphis indicates great variability in A.M. and P.M. urine concentrations. It must be recognized that there are different uses of urine PNP data. Spot samplings have been primarily used to detect contamination. However, the lack of correlation with absorption because of different exposure patterns. This issue needs to be addressed in a proposed 7-day study collecting A.M. and P.M. samples to determine variability. People with different exposure scenarios should be selected and the variability specifically assessed. It is also possible that the peak samples may prove useful to establish better cutoff points by offering better correlation with dose and any long-term subchronic effects.The “ideal” collection would be a pooled 24-hour sample: a complete collection of urine voided over a 24-hour period with testing of the PNP concentrations excreted during individual time intervals, and assessment of the PNP excretion over the 24 hours. The work group strongly recommends that the steering committee initiate a minimum 7-day study of urinary PNP biomonitoring (A.M. and P.M. minimal; 24-hour ideal) concurrent with environmental assessment to examine this issue. This pilot study will help the steering committee determine the minimal number of samples and sampling frequency required to assess overexposures using urinary PNP biomonitoring. The timing of the spot urinary PNPs should be based on the individual exposure questionnaire. Adjusting the spot urine test results for creatinine may reduce variability as described below. The usefulness of this adjustment can also be assessed in the 7-day study.
- The work group suggests that urine concentrations of PNP be corrected by urine creatinine concentrations to better reflect true MP clearance. The optimal approach would be to normalize creatinine to sex and body surface area as is done by clinical pharmacologists/nephrologists when dosing patients who have wide ranges of renal functions. In the case of MP, the justification is to correct for individual differences in urine concentration due to either homeostasis or renal disease where low PNP concentrations may be due to age or dilute urine rather than to low exposure to MP. The steady state production and excretion of creatinine into the urine may help normalize these variables.
What is the appropriate length of time to conduct biomonitoring to assess continued exposure? For susceptible populations, should biomonitoring occur beyond the time period for nonsusceptible populations?
The work group reiterates that both the frequency of biomonitoring and duration of biomonitoring should be based on field data rather than simply theoretic considerations. The environmental sampling data collected to date, when plotted against time since application, appear to show a long environmental half-life (227 days). Available data should be examined to see if similar temporal associations exist using urinary PNP as a measure of exposure, and how declines in urinary PNP relate to natural or clean-up related declines in indoor MP contamination. The current proposal to monitor at least 1 year appears to be a reasonable minimum. Longer biomonitoring may be required if accumulating data indicate the potential for the biomonitoring schedule to miss overexposure situations even after 1 year of followup. The estimates of variability that will be provided by the 7-day study will also help to address this issue .
Another important scheduling issue regarding the biomonitoring program is the frequency of sampling. If the 7-day study indicates high variability among spot urinary PNP samples, then the frequency of sampling may need to be increased. If more frequent urinary PNP samples need to be taken, it is preferable to weight these earlier in the course of biomonitoring to minimize continuing exposures due to misclassification errors. Finally, the work group concurs with the concept that high-risk populations may require more intensive biomonitoring (longer and perhaps more frequent); again, these decisions should be based on the considerable amount of data that is being accumulated.