Human Health Risk Assessment of Trace Chemicals in the Residential Environment Using Probabilistic Techniques

Abstract

Humans are exposed to a wide range of chemicals in the environment which may be hazardous to health. In this thesis two groups of chemicals from different mediums were investigated namely disinfection by-products (DBPs) in chlorinated drinking water and volatile aromatic hydrocarbons (VAHs) in residential indoor air.

The level of risk to human health was evaluated by health risk assessment using probabilistic techniques. In this thesis three factors were evaluated, (1) the exposure dose (EXPD) based on human exposure data (2) threshold Human Equivalent Dose (HED) based on test results on surrogate animals and (3) the dose-response using epidemiological data on adverse health responses as related to the threshold Lifetime Average Daily Dose (LADDH). The values obtained were plotted as cumulative probability (CP) relationships. The risk was characterized by three approach, (1) by comparison between EXPD with threshold HED and threshold LADDH, (2) by calculating the hazard quotient at 95/5 (HQ95/5) which was derived as the high exposure population from 95% from the cumulative probability of exposure doses (CP-EXPD) with the sensitive populations 5% from the human dose-response in terms of threshold cumulative probability Lifetime Average Daily Dose (CP-LADDH) and (3) by using hazard quotient by Monte Carlo techniques (HQMC) using Monte Carlo simulation via Crystal Ball software. Literature data on exposure concentrations from more than 10 countries and the adverse effects on animal laboratory experiments in terms of HED as well as human exposure and epidemiological studies in terms of LADDH were used in these evaluations.

With DBPs in chlorinated drinking water, EXPD was calculated from literature data from more than 10 countries and compared with threshold HED values from surrogate animals representing the threshold dose-response values for adverse effects. The highest human EXPD was lower than the threshold HED for TCM, BDCM, DBCM, TBM, DCAA and TCAA. All the threshold HED values were approximately 102 to 104 higher than EXPD at the 95% level. However, with the human epidemiology data, there was an overlap between the highest EXPD and the threshold LADDH representing the threshold dose-response values for adverse effects for TTHM, TCM, BDCM, DCAA and TCAA. This suggests that there are possible adverse health risks such as cancers and developmental effects on humans. The HQ95/5 values for TTHM, TCM and BDCM are 4.7, 5.7 and 9.2, respectively. This suggests that 5% of the human population has a high risk of adverse health effects. A similar result was also obtained for HQMC where the HQMC for TTHM (0.30>1), TCM (0.13>1) and DBCM (0.41>1) are higher than the unity value.

In the evaluation of VAHs from literature data, the comparison between the highest EXPD with the lowest threshold HED for adverse effects using surrogate animal data for ethylbenzene, mp-xylene, o-xylene and styrene shows no overlap. The lowest thresholds HED of these compounds are approximately 10 times higher than the EXPD at the 95% level. However, the maximum EXPD exceeded the threshold HED of benzene, toluene and naphthalene at the 60%, 94% and 30% respectively. Comparison was made using human exposure and human epidemiological data. The maximum EXPD exceeded the threshold LADDH derived from human health data for benzene and toluene only. This suggests that there are possible adverse health risks such as leukemia, developmental effect, aplastic anemia and chromosome aberration in blood. The HQ95/5 for benzene, toluene and styrene was 22.2, 1.2 and 0.3, respectively suggesting possible adverse effects in the 5% of the population.

In addition, the measurement of VAHs in the residential indoor air was also conducted for 32 houses in Brisbane. The total VAHs (TVAH) levels ranged between 2 to 137 g/m3 and were lower than the literature data on houses in Melbourne and Perth. The VAHs concentrations from indoor air of houses in Brisbane are higher than the outdoor ambient air. With an internal garage, the VAHs concentration is higher than the indoor air of the living room by 25 to 50% and may be a source of indoor VAHs due to the presence of motor vehicle in the garage. The age of the house was found to be positively related to VAHs concentrations in the house. The half-life of TVAH was found to be approximately 13 years.

The health risk assessment of VAHs in the indoor air of Brisbane houses was conducted using similar techniques to those used with the literature data for DBPs and VAHs. There was no overlap between maximum EXPD and thresholds HED dose-response data for all compounds except benzene and naphthalene. The lowest threshold HED for toluene, ethylbenzene, mp-xylene and o-xylene and styrene were found to be higher than the EXPD at 95% level by approximately 10 to 102. With benzene and naphthalene the maximum EXPD exceeded the threshold HED at the 95% and 85% respectively. An overlap was observed between maximum EXPD and threshold LADDH derived from human data for benzene only. The HQ95/5 for benzene, toluene and styrene are 2.7, 0.1 and 0.01 respectively. The HQ95/5 for benzene suggests 5% high exposure group in the human population has a high probability of risks of leukemia, developmental effect and aplastic anemia.