2 Exposure Assessment
Three exposure groups need to be considered in assessing the risk associated with a permethrin treatment concentration of 0.125 mg/cm 2 in fabric used to manufacture military battle-dress uniforms. These groups are (1) military nonfield personnel, (2) military field personnel, and (3) garment workers. The same basic information on carcinogenic potency can be used for all three groups, but with individualized exposure criteria being applied.
According to the U.S. Army (1993), nearly all permethrin lost from uniforms is lost in wash water, and little is transferred to other clothing laundered with the treated uniforms. Furthermore, only modest losses of permethrin from uniforms stored at extreme temperatures have been observed, suggesting low loss by volatilization in drying (U.S. Army, 1993). Thus, there appears to be little potential, if any, for either military or civilian personnel to become exposed as a result of the process of laundering impregnated uniforms.
MILITARY NONFIELD PERSONNEL
Because of permethrin's low volatilization, the U.S. Army and the subcommittee, considered the dermal route to be the only relevant route of exposure from impregnated uniforms. In a review of the Army's exposure assessment, EPA (1990a) concurred with the Army's position that
inhalation exposure to uniform wearers would probably be insignificant. At present, there is no information to indicate that exposure to uniform wearers will occur by any route other than dermal absorption.
Several conversion factors were used by the Army to translate the targeted fabric impregnation concentration, 0.125 mg/cm2, to an internal dose for military personnel through dermal absorption. These factors were the time-weighted-average percentage of permethrin remaining in fabric through 50 washings (26%), percentage of migration from fabric to skin (0.49% per day; Snodgrass, 1992), body contact area (1.5 m2), dermal absorption rate (2% per day; Bartelt and Hubbell, 1987), and adult body weight (70 kg). These conversion factors were reviewed by EPA (1990b). The only factor that was questioned by EPA was the assumed human dermal absorption rate of 2% per day for permethrin after being transferred from clothing to skin. EPA indicated that this rate should be increased from 2% to 7%, which would increase the Army's calculated exposure dose by a factor of 3.5. However, EPA's estimate of dermal absorption factor is based on studies of dermal absorption of permethrin in rabbits, whose skin is much more permeable to chemicals than human skin. The California EPA (CEPA), like the Army, used a dermal absorption rate of 2% for a 24-hr period (CEPA, 1992). This rate was established on the basis of a review of various in vivo and in vitro studies of permethrin applied to human skin (see Chapter 3, “Dermal Absorption,” for details). Based on the available data cited by CEPA, the Army 's assumed dermal absorption rate of 2% per day appears defensible.
To adjust for actual exposure conditions, the Army assumed that military personnel would wear the treated uniforms 16 hr per day for 6 years during a 75-year lifetime. Adjusting for the proportion of lifetime exposed resulted in a calculated average daily lifetime dose of 3.6 × 10−5 mg/kg per day. (Appendix A provides a detailed method for calculating exposure dose from wearing BDUs.) The Army has since indicated that an exposure scenario of 18 hr per day for 10 years should be used instead of 16 hr for 6 years (U.S. Army, 1993). That revision increases the Army's calculated average daily lifetime dose by a factor of 1.9, resulting in a final value of 6.8 × 10−5 mg/kg per day.
MILITARY FIELD PERSONNEL
The same considerations that led to calculation of the permethrin-exposure dose for military nonfield personnel would apply to field personnel. The only difference between field and nonfield personnel is that field troops apply DEET topically to areas of the skin not normally covered by the treated uniform. The application of DEET raises the possibility of DEET's facilitating the absorption of permethrin through the skin where DEET and permethrin overlap.
At present, there are insufficient data to evaluate the potential for DEET to enhance permethrin absorption. However, somewhat less than 5% of the skin would be expected to have overlapping exposure to DEET and permethrin. Thus, at present, no adjustment is being made to distinguish exposure patterns for military field and nonfield personnel. If subsequent data indicate that exposure to both permethrin and DEET is an important consideration, then the exposure assessment for field personnel can be modified.
Estimation of dermal exposure to garment workers from handling impregnated fabric in the manufacture of uniforms requires that some of the factors used to convert the target impregnation concentration, 0.125 mg/cm 2, to an internal dose be different from those used for military personnel. Since washing of the uniform is not a consideration, it is assumed that 100% of the permethrin is available for transfer to the skin. The migration rate from fabric to skin is assumed to be the same as that for uniform wearers, 0.49% per day; although garment workers might handle many different pieces of fabric during a day, it is assumed that contact would be similar to contact with the same piece of fabric, as a uniform wearer would have. The body-surface area, hands and forearms, is conservatively assumed to be 1,600 cm2, which is 10% of the body-surface area of an average female garment worker. The dermal absorption rate is assumed to be 2% per day, as it is for uniform wear-
ers. Adult female body weight is assumed to be 60 kg. According to a potential manufacturer of treated uniforms, hands and forearms of workers would be exposed to treated cloth 8 hr per day, 250 days per year (U.S. Army, 1993). It is assumed that exposure would last 30 years on average.
Putting all these factors together gives the average daily lifetime internal dose as
(0.125 mg/cm2 × 0.0049/day × 1,600 cm2 × 0.02/60 kg) × (8/24) × (250/365) × (30/75) = 3.0 × 10−5 mg/kg/day.
That dose is for dermal exposure to permethrin from direct contact only with treated cloth.
Depending on the degree of ventilation and dust removal in the cutting and sewing processes and the type of protective clothing and equipment worn by garment workers, airborne particles could constitute an additional source of exposure to permethrin. Such exposure could occur by inhalation and ingestion or by ocular and dermal routes. Thus, the above estimated internal dose of 3.0 × 10−5 mg/kg per day possibly represents a lower bound on the overall exposure of garment workers to permethrin. The subcommittee could not provide an estimate of the upper bound on permethrin exposure because data are not available on other potential routes of exposure. The lower bound might actually be an upper bound if other routes do not come into play.
The subcommittee recommends that studies should be conducted to collect data on the representative permethrin exposure factors to produce a more complete and accurate risk characterization for garment workers.