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Research paper : A strategic approach for comparing different types of health risks (A. Kishimoto)−34−Synthesiology - English edition Vol.1 No.1 (2008) lognormal distribution, which was called the (A) 24-hour average inter-house variability. To estimate (C) annual average inter-house variability, we designed the following procedures.First, we estimated the variability of the daily average “indoor concentrations of indoor origin” in a house, i.e. (B) 24-hour average within-house variability. Next, we assumed that (A) equaled (B) plus (C), i.e. (C) would be obtained by subtracting (B) from (A). It was assumed that “indoor toluene concentrations of indoor origin” were proportional to the emission rate from indoor sources and inversely proportional to the residential air exchange rate (AER). However, we could not find the annual data for either emission rates or AER. After expert interviews, we judged that the daily variability throughout the year for toluene emission rates from indoor sources was virtually zero under all possible temperature ranges, and that the daily variability throughout the year for 24-hour average AER ranged from 0.5 times per hour to 10 times per hour (range in which 95 % of the values fell). Assuming these variabilities were all independent of each other, the annual average “indoor toluene concentrations of indoor origin” could be described as lognormal distribution whose geometric mean was 15.72 µg/m3 and geometric standard deviation was 4.28. The distribution of personal exposure was calculated as a weighted average of indoor and outdoor concentrations, applying the result of a time-budget survey that showed that Japanese residents spent approximately 90 % of their day indoors and 10 % outside [4]. Figure 4 shows that the distribution moves from left to right with addition of each emission source category. 3.4 Derivation of dose response functions from epidemiological studyThe goal of toxicity assessment in conventional risk assessment is to derive NOAEL from animal tests or epidemiological studies and set reference values. However, since the type and severity of toxicity endpoint and the extent of conservativeness (extent of overestimation) are different for different chemical substances, it is difficult to compare the risks from the NOAEL or reference values. We attempted to derive the relationship between health effects and exposure levels, i.e. dose response functions, from the results of an epidemiological study [5]. Among various subjective symptoms investigated in the epidemiological study in which subjects were workers at gravure printing factories, eight symptoms that were found to have significant associations between exposure levels and incidence rates were selected, and their dose response functions were derived as shown in Figure 5. The vertical axis indicates the incidence rates, and the horizontal axis shows the exposure levels. As the exposure increased, probabilities of incidence for each symptom as well as the expected number of symptoms increased.3.5 Quantification of human health risksThe distribution of personal exposure concentrations (Section 3.3) was substituted into the dose response functions (Section 3.4), and the incidence numbers obtained were then multiplied by their severity. The health risks for residents in Japan from exposure to toluene were finally quantified as total loss of QALYs. The severity of each symptom and some combinations of symptoms were expressed in terms of QOL index, where 0 meant death and 1 meant normal health. QOL indices were calculated based on the Health Utilities Index Fig. 4 Distribution of personal exposures (annual average) of Japanese residents* “High-emission facilities” indicates those emitting more than 300,000 tons per year.Common logarithm of the distribution of “indoor concentration of indoor origin” (µg/m3)Breakdown of 24 hsDistribution of toluene emissions Number of people [10,000]Exposure range [µg/m3]Indoor sources (A)+ Mobile and low-emission sources (A+B)+ High-emission facilities (A+B+C)05001,0001,5002,0002,5003,0003,50000~2~4~8~16~32~65~130~260~520~1,040~2,080~4,160~10,000~-2~-1.5~ -1~-0.5~0~0.5~1~1.5~2~2.5~3~3.5~4~01020304050607.4E+00~4.1E-01~2.2E-02~1.2E-03~6.7E-05~3.7E-06~2.0E-07~0.0E+00~ [g/sec]Indoor,90 % Outdoor,10 % Frequency [cases]

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