Populations in areas that contain hazardous waste sites may be exposed to dieldrin for brief periods. Exposure would most likely occur by the inhalation or oral routes, but dermal exposure is also possible. There are acuteduration oral exposure data in humans from cases of accidental or intentional poisonings that indicate that the central nervous system is a major target organ of dieldrin toxicity by the oral route. Convulsions have been observed following ingestion of very high concentrations of dieldrin.
Also, acute oral exposure in humans has been reported to cause renal toxicity. Renal toxicity has not been reported in studies in animals after acuteduration ingestion of high concentrations of dieldrin, however, the number of studies examining systemic effects associated with acuteduration exposures is quite limited. Studies in laboratory animals examining the effects of ingestion of dieldrin have supported the conclusion that the nervous system is a major target organ of dieldrin toxicity. In such studies, convulsions as well as impaired responding in operant behavioural paradigms were reported. In addition, immune suppression, and adaptive changes in the liver have been observed in acutely exposed animals. Results of these studies indicate that the immune system may be the most sensitive target organ for the effects of brief oral exposures to dieldrin. An acuteduration oral MRL was not derived for dieldrin because the database indicates that the most sensitive target of toxicity is the immune system in rats administered acute doses of dieldrin and there are no data to suggest that the immune system may be a target of toxicity in humans following ingestion of dieldrin.
No information is available regarding acuteduration inhalation exposure to dieldrin in humans, and extremely limited information is available from studies in animals. Although the volatility of dieldrin is quite low and levels in the atmosphere are expected to be quite low, absorption of these compounds by the lungs occurs to a significant extent. Toxicokinetic data do not indicate that dissimilar target organs would be affected as a result of inhalation exposure to dieldrin.
Information regarding the acute effects of dermal exposure of dieldrin is limited to lethality studies in animals. Dermal exposure to dieldrin is possible in contaminated soil, and toxicokinetic studies indicate that dermally applied dieldrin is absorbed. Toxicokinetic data do not suggest that dissimilar target organs would be affected as a result of dermal exposure.
The World Health Organization has not listed dieldrin in WHO Acute Hazard Rankings because as an active ingredient is believed to be obsolete or discontinued for use as a pesticide.
Studies performed by the U.S. EPA placed dieldrin formulations in Acute Toxicity Rankings in a Category 1 and 2, Moderately to Highly Toxic (Table below).
Few reports were located regarding effects in humans after intermediateduration exposure to dieldrin by any route. These studies showed that the nervous system is a major target organ in humans after intermediateduration exposures. Studies in laboratory animals confirm this observation. Other targets identified in intermediateduration oral studies in animals include the immune system, the developing neonate, the reproductive system, the kidney, and the liver.
An intermediateduration oral MRL was developed for dieldrin based on a NOAEL for impaired learning in monkeys.
No data were located regarding intermediateduration inhalation exposures in animals, and human exposure levels were not quantified. Therefore, no intermediateduration inhalation MRL was derived for dieldrin. Also, only limited information was located regarding lethality, neurological effects, and dermal effects after intermediateduration dermal exposures. As noted above, absorption occurs by both the inhalation and dermal routes, and toxicokinetic data indicate that similar target organs would be affected following exposure to either route.
A number of epidemiological studies have been conducted on workers exposed chronically to dieldrin. In these studies, doses are usually not well quantified, and concomitant inhalation, dermal, and possibly oral exposures have occurred.
Data from the existing epidemiological studies indicate that the nervous system is a major target organ for chronic inhalation, dermal, and possibly oral exposures in humans. Chronic oral studies in animals also indicate that the nervous system is a major target organ, but additionally demonstrate adverse effects of dieldrin on the kidney and liver. The liver was the most sensitive target of toxicity in chronicduration studies and hepatic effect levels in rats were used as the basis of chronic oral MRLs for dieldrin. No chronic animal studies were located for the inhalation route. One animal study was located examining the effects of chronic dermal exposure. Studies examining the effects caused by lowlevel chronic exposures by both the inhalation and oral routes would be valuable for determining whether such exposures could cause toxicity in populations exposed to dieldrin near hazardous waste sites for extended periods.