p-Dichlorobenzene Risk Assessment Document

ABSTRACT

p-Dichlorobenzene (1,4-dichlorobenzene; CAS Registry No. 106-46-7, hereafter referred to as pDCB) was first produced commercially in the U.S.A. in 1915 (IARC, 1982). It has long been used as a moth repellent and as a space deodorant; it is also used industrially as an intermediate in organic syntheses.

The pDCB that is found in indoor air originates mainly from moth repellents used to protect clothing and from deodorants that are used in the household. 

The pDCB that is existing indoor, while providing benefits such as moth control and deodorizing effects, raise some concerns regarding potential risks to human health. We focused our risk assessment on human exposure through inhalation of pDCB in indoor air because pDCB is mainly used as a moth repellent to protect clothing,. 

Our primary objective was to estimate the magnitude of risks from the present uses of pDCB by examining and analyzing knowledge regarding exposures and potential hazards. 

Our secondary objective was, in the event of a significant risk being identified, to suggest possible actions that might be adopted, taking into account the risk–benefit tradeoff. In estimating the risks, we investigated whether hazards observed in animal experiments were likely to occur in humans, and we estimated the distributions of mid- to long-term exposure concentrations from monitoring data, which were derived from short-term measurements.

This document contains a comprehensive review of hazard-assessment information, including oral exposures, and summarizes its rationale in the reference concentration, for chronic inhalation hazards, which was determined by Research Center for Chemical Risk Management, National Institute for Advanced Industrial Science and Technology (CRM, AIST). 

The endpoint of the assessment of the toxicity hazard of chronic exposure to pDCB was hepatotoxicity. Hepatotoxity was observed in test animals exposed to pDCB by both oral and inhalation routes; increases in liver weight, increases in leakage of enzymes in the liver, and histopathological changes were also observed. 

After results of some chronic inhalation studies were examined in detail, a high-quality two-year study in mice was chosen as the study on which to base the reference concentration. 

Non-neoplastic changes observed in the livers of mice in this study were defined as an endpoint, and the no observed adverse effect level (NOAEL) was calculated to be 80 mg/m3 by converting the study result of 75 ppm into an equivalent concentration at a continuous exposure of 24 hours/day. 

The reference concentration was determined to be 800ƒÊg/m3; this value was obtained from the value of the NOAEL (80 mg/m3) divided by an uncertainty factor (100).

In this document, the amount of the pDCB emitted to the envirionment, pDCB concentrations in indoor air and outdoor air, and pDCB concentrations in the environment re summarized. In order to obatining the distributions of pDCB concentrations of indoor air, raw data from "Nationwide Survey on Volatile Organic Compounds in Indoor Air" (Ministry of Health and Welfare 1999) were analyzed. 

From the result of this analysis, the proportion of households using pDCB-based moth repellents/fumigants was determined to be 0.9 and the proportion of households not using it was 0.1 (this group of households was called "households with no indoor usage" (abbreviated as N)). 

Furthermore, according to the histogram for concentrations contributed by indoor sources of emission ([indoor concentration] – [outdoor concentration]), households using pDCB-based moth repellents/fumigants could be classified into “households with higher indoor usage” and “households with lower indoor usage” (ratio 0.61:0.39). 

By assuming that each group has a lognormal distribution, a geometric mean (GM) and a geometric standard deviation (GSD) were obtained followed by goodness-of-fit test to the histogram. Thus, distributions of concentrations contributed by indoor emission sources in "living spaces"  were obtained for each of the three groups "households with higher indoor usage" (abbreviated as H), "households with lower indoor usage" (abbreviated as L), and "households with no indoor usage" (N). 

The GM (ƒÊg/m3) values were 60.7, 2.1, and 0, respectively, and the GSD values were 5.1, 3.2, and 1, respectively; the ratio of the three types of household groups was 0.55:0.35:0.10. 

Next, distributions of concentrations (arising from indoor sources of emission) throughout indoor spaces, including bedrooms, were estimated under the assumption that concentration of a bedroom is an average of two times as a living room at the house. 

Thus, the average concentration within a house was calculated, taking into account the time that individual family members spent indoors. If the total indoor concentrations should be obtained, Outdoor concentrations were added to these concentrations.

Human exposure to pDCB was evaluated on the basis of indoor concentrations and outdoor concentrations, taking into consideration the time spent in each situation. 

The exposed population was divided into two groups according to their patterns of life spent. One group consisted of housewives, babies and preschool children, and elderly persons (abbreviated LT, see Table 3); the other group consisted of working people and students (ST). 

The exposed population was also divided into three groups depending on the amount of pDCB used in the household. These groups were "households with higher indoor usage" (H), "households with lower indoor usage"  (L), and "households with no indoor usage" (N). As a result, the population as a whole was classified into a total of six groups, i.e. LT & H, LT & L, LT & N, ST & H, ST & L, and ST & N. By using this distribution together with the outdoor concentrations obtained in Chapter V, inhalation exposure levels were estimated for the above six groups, and rates of exposure exceeding the reference concentration were identified.

The proportion of the population for which the exposure concentration exceeded the reference value of 800ƒÊg/m3 was 5.4% for the group with the highest exposure (the group LT & H, Table 5). 

The ratio for the population as a whole was calculated to be 2.4%, half of which belonged to the group LT & H; the other half belonged to the group ST & H (Table 6). People in these groups need to take some measures to reduce their exposure to indoor concentrations of pDCB.

Summary [PDF file, 233KB] 24 March 2006