Estimation of
Dioxin Exposure Concentrations and
Dioxin Intakes of Workers at
Continuously Burning Municipal Waste Incinerators
Journal of Occupational Health v.43, n.2, Mar01
Shinji KUMAGAI1, Shigeki KODA2, Takashi MIYAKITA3, Hideki YAMAGUCHI4, Kenichi KATAGI5 and Mitsuo UENO6
J Occup Health 2001; 43: 61-69
1 Department of Occupational Health, Osaka Prefectural Institute of Public Health,
2 Department of Public Health, Kochi Medical School,
3 Department of Hygiene, Kumamoto University School of Medicine,
4 Kumamoto Occupational Safety and Health Center,
5 Kobe Hospital, Hyogo Medical Cooperation and
6 Institute of Occupational Safety and Health, All Japan Prefectural and Municipal Workers' Union
Abstract:
Estimation of Dioxin Exposure Concentrations and Dioxin Intakes of Workers at Continuously Burning Municipal Waste Incinerators: Shinji KUMAGAI, et al. Department of Occupational Health, Osaka Prefectural Institute of Public Health—In order to examine the dioxin exposure level in incinerator workers, total dust concentration in the breathing zone and dioxin concentrations in fly ash, slag and deposited dust were determined at three municipal waste incineration plants A, B and C equipped with continuously burning incinerators of stoker type. Three incinerators were operated at plants A and B, and four at plant C. The incinerators of plants A and B were equipped with electrostatic precipitators for removing particulate matter from the flue gas stream. In plant C, three incinerators were equipped with electrostatic precipitators and one with a bag filter. Total dust exposure concentrations during daily operation of the incinerators were 0.11 to 1.50 mg/m3, and dioxin concentrations in the deposited dust were 1.0 to 6.4 ng toxicity equivalent (TEQ)/g. Dioxin exposure concentrations were estimated to be 0.5 to 7.1 pg TEQ/ m3. Total dust exposure concentrations during periodic cleaning of the inside of the incinerator were 30 to 780 mg/m3, and dioxin concentrations in the slag were 0.004 to 1.1 ng TEQ/g. Dioxin exposure concentrations were estimated to be 0.5 to 48 pg TEQ/m3. Total dust exposure concentrations during periodic cleaning of the inside of the electrostatic precipirator were 51 to 2,000 mg/m3, and dioxin concentrations in the fly ash were 7.3 to 64 ng TEQ/g. Dioxin exposure concentrations were estimated to be 370 to 92,000 pg TEQ/m3, which were 150 to 37,000 times as high as the dioxin administrative level (2.5 pg TEQ/m3). Total of occupational and environmental dioxin intakes for an incinerator worker is predicted to exceed the tolerable daily intake (TDI: 4 pg TEQ/kg/d), when dioxin concentration in fly ash is 1,000 ng TEQ/g, even if an airline mask is used during the periodic maintenance. Consequently, it is essential to decrease the dioxin concentration in fly ash. When the dioxin concentration in fly ash is no higher than 100 ng TEQ/g, if an airline mask is used during the periodic maintenance, the dioxin intake is predicted to be lower than the TDI. Thus, wearing an airline mask would be effective. (J Occup Health 2001; 43: 61-69)
Key words: Dioxin, Incinerator worker, Dust exposure, Dioxin daily intake
Received Sept 4, 2000; Accepted Nov 13, 2000 Correspondence to: S. Kumagai, Department of Occupational Health, Osaka Prefectural Institute of Public Health, 1-3-69, Nakamichi, Higashinari-ku, Osaka 537-0025, Japan
Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and coplanar polychlorinated biphenyls (co-PCBs) are chemically and biologically similar compounds and also highly toxic chemicals. "Dioxins" is the general term for these chemicals. They already contaminate the environment, i.e., the air, water and soil, and foods, including fish, meat and vegetables[1-3]. Dioxins are also found in human adipose tissue, blood and milk[1, 4-6]. The main sources of dioxins are production of organic chlorinated herbicides, bleaching of paper or pulp and incineration of waste7-9). In Japan, the major source is incinerators because most solid waste is incinerated in municipal waste incinerators without sufficient measures to prevent the generation of these chemicals[10]. This led to concern about adverse health effects on workers employed at incineration plants as well as residents living in neighboring areas. In a municipal waste incineration plant equipped with 16-h burning incinerators of fluidized bed type, the mean concentration of PCDDs and PCDFs in blood was 323 pg toxicity equivalent (TEQ)/g lipid among the highly exposed worker group[11]. This value was about 15 times as high as that of the general population. In three municipal waste incineration plants equipped with day and night continuously burning incinerators of stoker type, the TEQ values of PCDDs and PCDFs in serum did not significantly increase among the incinerator workers, but the concentrations of 1,2,3,4,6,7,8-heptachloro dibenzofuran in serum significantly increased, which suggests that these incinerator workers had inhaled dust containing PCDDs and PCDFs during their work[12]. Thus, the dioxin exposure conditions of the incinerator workers should be clarified. In this study, total dust concentration in the breathing zone of the incinerator workers and dioxin concentrations in the fly ash, slag and deposited dust were determined, and the dioxin exposure concentration was estimated. The daily dioxin intake of an incinerator worker was also estimated.
Table 1. Outline of the three municipal waste incineration plants
Incineration plant A B C . Number of incinerators 3 3 4 Type of incinerator Stoker Stoker Stoker Operation hours (h/d) 24 24 24 Cooling of exhaust fumes Water Water Boiler shower shower Equipment for removing dust EP*1 EP*1 EP*1,BF*2 from exhaust fumes Volume of incinerated waste 160 300 600 (tonnes/d) *1: Electrostatic precipitator, *2: Bag filter
Methods
Incineration plants
This study was conducted at three municipal waste incineration plants A, B and C in 1998-99. The characteristics of these plants are presented in Table 1. Three incinerators were operated at plants A and B, and four at plant C. All incinerators were day and night continuously burning and stoker type. The stoker is a device for conveying waste on the floor of the incinerator. The combustion gas from the incinerator was cooled by water shower at plants A and B and by a boiler at plant C. The incinerators of plants A and B were equipped with electrostatic precipitators (EP) for removing particulate matter from the flue gas stream. At plant C, three incinerators were equipped with EPs and one with a bag filter (BF). Acids such as hydrogen chloride were removed by passing through a layer of granular calcium oxide in the toxic gas scrubber in plant A and by spraying alkaline solution in plants B and C. The volumes of incinerated waste were 160, 300 and 600 tonnes/day at plants A, B and C, respectively. In 1996-97, the concentrations of PCDDs and PCDFs in the exhaust gas of the incinerators with electrostatic precipitators were 8.9 to 42, 0.82 to 1.4 and 3.1 to 12 ng TEQ/Nm3, respectively, while that of the incinerator with a bag filter was 0.07 to 0.24 ng TEQ/Nm3.
Incineration process and tasks of incineration workers Figure 1 shows the incineration process in plant A. First, the collected waste is weighed and put in the waste pit. Next, the waste is transported into the incinerator by the crane. In the incinerator, the waste is dried and burned while being conveyed to the bottom by the stoker. The combustion gas is passed through the cooling tower, EP and toxic gas scrubber, and emitted from the chimney. Slag remaining on the bottom of the incinerator and fly ash collected by EP are transported to the ash pit. The slag and fly ash accumulated in the ash pit are loaded onto a truck and carried to reclaimed ground. In this process, when the conditions of combustion and cooling are not appropriate, dioxins are synthesized in the incinerator, cooling tower and EP, and contaminate the slag, fly ash and exhaust gas. The dioxins also contaminate the granular calcium oxide for plant A and alkaline solution for plants B and C in the toxic gas scrubber.
The tasks of incinerator workers can be classified into "daily operation" and "periodic maintenance." The daily operation consists of weighing waste, crane operation, incinerator operation, carrying slag and fly ash, and daily inspection and maintenance. Each incinerator is stopped every 3 to 4 months and periodic maintenance is conducted. First, the insides of the incinerator, cooling tower, EP and BF are cleaned. The inside of the toxic gas scrubber is also cleaned by the incinerator workers in plant A, but by workers employed at other companies in plants B and C. Next, these apparatuses are inspected and repaired by the outside workers in all the plants.
Measurement of total dust exposure concentration
Total dust in the breathing zone of an incinerator worker was collected on a glass fiber filter (T60A20, 25 mmΦ or 35 mmΦ, Shibata, Japan) with a portable sampling pump (MP-2N, Shibata, Japan and GILAir 5, Gilian, USA) at 1 to 2 L/min. The filter was weighed using a balance (H51, Mettler) before and after the sampling, and total dust exposure concentration was calculated. The sampling was done during the working hours for the daily operation and during the task period for the periodic maintenance. When the dust concentration was very high, the filter was replaced with a new one during the sampling.
Measurement of dioxin concentrations in fly ash, slag and deposited dust
In order to estimate the dioxin concentration of dust in the breathing zone of the workers who did the daily operation, samples were taken from the dust deposited on beams in the incineration plants. Also, in order to estimate the dioxin concentration of dust in the breathing zone of the workers who did the periodic maintenance, samples were taken from the fly ash remaining in the stopped EPs and from the slag remaining in the stopped incinerators. Fine particles are expected to exist in the breathing zone, because they can be easily scattered by disturbance and remain suspended in air for a long time, while coarse particles cannot. Though the size of the particles suspended in the breathing zone varies due to the situations, it may be 100 µm or less[13]. In this study, the samples of the deposited dust, fly ash and slag were passed through a sieve of 32 µm, and the dioxin concentrations were determined according to a manual of the Ministry of Welfare[14]. Briefly, the sample was first treated with 2 N HCl for better extraction efficiency, and the solid content was separated with a filter, washed with deionized water, and then dried at room temperature. Dioxins in the liquid passing through the filter were extracted twice with toluene, and those in the solid were Soxhlet-extracted for 16 h with toluene. The two toluene extracts were combined and washed with deionized water. The extract was dried and reconstituted to hexane, and the internal standard (13C12-2,3,7,8- substituted dioxins) was added. The extract was cleaned with a multistage column of silica gel coated with AgNO3, H2SO4 and KOH, and an aluminium oxide column. The effluent was dried and reconstituted to nonane containing 13C12-1,2,3,4- tetrachlorodibenzo-p-dioxin. Finally, 2,3,7,8-substituted PCDDs and PCDFs were quantitatively determined using a high-resolution gas chromatograph (HP5890II, Hewlett Packard, USA)/high-resolution mass spectrometry (JMS700, JEOL, Japan) connected with capillary columns of SP2331 (0.25 mm I.D. × 60 m, Supelco, USA) or DB5- MS (0.25 mm I.D. × 30 m, J & W, USA). The TEQ values were calculated according to the World Health Organization[15].
The term "dioxins" includes co-PCBs as well as PCDDs and PCDFs by the definition in use at the present time. However, they were not included in the term when we conducted this study. Thus, concentrations of co- PCBs in the deposited dust, fly ash and slag were not determined.
Estimation of dioxin intake by an incinerator worker
Dioxin intake by an incinerator worker was estimated based on the following assumptions. The body weight is 60 kg and the respiratory ventilation is 1 m3/h. In one year, there are 250 working days and the periodic maintenance is done 4 times. The worker inhales the same dust as the deposited dust during the daily operation, and the slag while cleaning the inside of the incinerator and cooling tower and the fly ash while cleaning the inside of the EP. As stated above, the inside of the toxic gas scrubber was cleaned by the workers employed at other companies in plants B and C, so that dioxin intake while cleaning the toxic gas scrubber was not included in this estimation. Dioxin concentration in the fly ash is assumed to be 1, 10, 100 or 1,000 ng TEQ/g. Dioxin concentrations in the slag and in the deposited dust are assumed to be 1/ 100 and 1/10 times, respectively, as high as that in the fly ash. Environmental intake, such as through meals and general air, is 2.60 pg TEQ/kg/day (PCDDs, PCDFs and co-PCBs), as shown by the National Environmental Council[16]. The protection factors of dust mask and airline mask are assumed to be 10 and 1000[17]. The dioxin intake was calculated according to the following equation.
DI = TD · D · R · W · N/365/BW/PF,
where DI = Dioxin intake
TD = total dust exposure concentration (mg/m3)
D = dioxin concentration in dust (ng TEQ/g)
R = respiratory ventilation (m3/h)
W = working duration (h/once)
N = number of times (times/yr, 200 for the daily operation, 4 for the periodic maintenance)
BW = Body weight (kg)
PF = Protection factor
Table 2. Dioxin concentrations in fly ash, slag and deposited dust (ng TEQ/g)
Incineration plant A B C .
Fly ash 7.3 46 64
Slag 0.26(0.04*1) 0.004(0.0001*1) 1.1 (0.02*1)
Deposited dust 4.8(0.66*2) 1.0(0.02*2) 6.4(0.10*2)
*1: Ratio of dioxin concentration in slag to fly ash.
*2: Ratio of dioxin concentration in deposited dust to fly ash.
The measured values do not include co-PCB concentration.
Table 3. Total dust exposure concentrations and estimated dioxin exposure concentrations of incinerator workers
Total dust Estimated dioxin
exposure exposure
concentration concentration Working
Sample (mg/m3) (pg TEQ/m3) duration
Plant number Mean Range Mean Range (h)
Daily operation A,C 8 0.39 0.11-1.50 2.0 0.5-7.2 8
Periodic maintenance (cleaning
of the inside of equipment)
1. Incinerator
I. Removing clinker A,C 6 55 30-97 29 9.2-48 1*
II. Removing clinker and B 3 420 130-780 1.7 0.5-3.1 4*
slag
2. Duct under stoker A,C 6 82 12-170 36 13-70 1*
3. Cooling tower A,B 5 210 53-420 48 0.2-110 1*
4. Electrostatic precipitator
I. Removing fly ash A 3 120 51-200 880 370-1500 0.5*
deposited on discharge position
II. Removing fly ash by B 2 1800 1500-2000 81000 71000-92000 1*
compressed air
*: Working duration in one periodic maintenance.
Fig. 1. Incineration process in plant A.
Results
Dioxin concentrations in fly ash, slag and deposited dust
Table 2 shows the TEQs of PCDDs and PCDFs in fly ash, slag and deposited dust. The dioxin concentrations in fly ash were 7.3, 46 and 64 ng TEQ/g, respectively, for plants A, B and C, those in slag were 0.26, 0.004 and 1.1 ng TEQ/g, respectively, and those in deposited dust were 4.8, 1.0 and 6.4 ng TEQ/g, respectively.
Total dust exposure concentrations and estimated dioxin exposure concentrations
Table 3 shows the total dust exposure concentrations and the estimated dioxin exposure concentrations. In the daily operation, the total dust exposure concentrations were 0.11 to 1.50 mg/m3 with a mean of 0.39 mg/m3. By multiplying the total dust exposure concentrations by the dioxin concentrations in deposited dust, the dioxin exposure concentrations were estimated to be 0.5 to 7.2 pg TEQ/m3.
For the periodic maintenance, two methods of cleaning the inside of the incinerator and EP were found in this study. At plants A and C, the workers removed glassy lumps (clinker) adhering to the wall with a stick in the incinerator and removed fly ash adhering to the discharge hole in the EP (method I). At plant B, the workers removed clinker and all slag remaining on the stoker by shovel in the incinerator and removed all fly ash adhering to the wall using compressed air in the EP (method II). In method II, the inside of the incinerator and EP were filled with slag and fly ash, and the total dust exposure concentrations were extremely high, 130 to 780 mg/m3 in the incinerator and 1,500 to 2,000 mg/m3 in the EP. On the other hand, in method I, the total dust exposure concentrations, while still very high, were about 1/10 those of method II, 30 to 97 mg/m3 in the incinerator and 51 to 200 mg/m3 in the EP.
During incineration, metals such as aluminum included in waste melt in the incinerator and drop through ducts under the stoker. However, some of the metals adhere to the wall of the ducts and must be removed by periodic maintenance. Also, slag adheres to the wall of the cooling tower and must be removed by periodic maintenance. The total dust exposure concentrations were 12 to 170 mg/m3 in the ducts and 53 to 420 mg/m3 in the cooling tower, which were very high levels.
By multiplying the total dust exposure concentrations by the dioxin concentrations in slag and fly ash, the dioxin exposure concentrations during the periodic maintenance were estimated to be 0.5 to 48 pg TEQ/m3 in the incinerators and 370 to 92,000 pg TEQ/m3 in the EPs.
Estimated dioxin intake
Table 4 shows the estimated dioxin intake by an incinerator worker not using respiratory protection devices. The total of occupational and environmental dioxin intakes were estimated to be 4.12 pg TEQ/kg/d in the case of fly ash of 100 ng TEQ/g for method I and to be 5.96 pg TEQ/kg/d in the case of fly ash of 10 ng TEQ/ g for method II, which exceed the tolerable daily intake (TDI: 4 pg TEQ/kg/d) recommended by the National Environmental Council[16].
Table 5 shows the estimated dioxin intake by an incinerator worker wearing a dust mask only during the periodic maintenance. The dioxin intakes were estimated to be 7.32 pg TEQ/kg/d in the case of fly ash of 1,000 ng TEQ/g for method I and to be 6.28 pg TEQ/kg/d in the case of fly ash of 100 ng TEQ/g for method II, both of which exceeded the TDI.
Table 6 shows the estimated dioxin intake by an incinerator worker wearing an airline mask only during the periodic maintenance. The dioxin intakes were estimated to be 6.17 pg TEQ/kg/d in the case of fly ash of 1,000 ng TEQ/g for method I and to be 6.50 pg TEQ/ kg/d in the case of fly ash of 1,000 ng TEQ/g for method II, both of which exceeded the TDI.
Table 4. Estimated daily dioxin intake by incinerator workers without respiratory protection devices*1
Fly ash (ng TEQ/g) 1 10 100 1000 Slag (ng TEQ/g) 0.01 0.1 1 10 Dust in workplace (ng TEQ/g) 0.1 1 10 100 Estimated dioxin intake (pg TEQ/kg/day) Occupational intake Daily operation 0.004 0.04 0.36 3.56 Periodic cleaning I*2 0.01 0.12 1.16 11.6 II*2 0.33 3.27 32.7 327 Environmental intake 2.60 2.60 2.60 2.60 (Food, General air)*3 Total I*2 2.61 2.76 4.12 17.8 II*2 2.93 5.91 35.7 333 *1: Body weight: 60 kg, Respiratory ventilation: 1 m3/h, Periodic cleaning: 4 times/yr, Working days: 250 d/yr. *2: See Table 3. *3: National Council for Environment.
Table 5. Estimated daily dioxin intake by incinerator workers with wearing a dust mask only during periodic maintenance*1
Fly ash (ng TEQ/g) 1 10 100 1000 Slag (ng TEQ/g) 0.01 0.1 1 10 Dust in workplace (ng TEQ/g) 0.1 1 10 100 Estimated dioxin intake (pg TEQ/kg/day) Occupational intake Daily operation 0.004 0.04 0.36 3.56 Periodic cleaning I*2 0.001 0.01 0.12 1.16 II*2 0.03 0.33 3.32 33.2 Environmental intake 2.60 2.60 2.60 2.60 (Food, General air)*3 Total I*2 2.61 2.65 3.08 7.32 II*2 2.63 2.97 6.28 39.4 *1: Body weight: 60 kg, Respiratory ventilation: 1 m3/h, Periodic cleaning: 4 times/yr, Working days: 250 d/yr, Protection factor: 10. *2: See Table 3. *3: National Council for Environment.
Table 6. Estimated daily dioxin intake by incinerator workers with wearing an airline mask only during periodic maintenance*1
Fly ash (ng TEQ/g) 1 10 100 1000 Slag (ng TEQ/g) 0.01 0.1 1 10 Dust in workplace (ng TEQ/g) 0.1 1 10 100 Estimated dioxin intake (pg TEQ/kg/day) Occupational intake Daily operation 0.004 0.04 0.36 3.56 Periodic cleaning I*2 0.00001 0.0001 0.001 0.01 II*2 0.0003 0.003 0.03 0.33 Environmental intake 2.60 2.60 2.60 2.60 (Food, General air)*3 Total I*2 2.60 2.64 2.96 6.17 II*2 2.60 2.64 2.99 6.50 *1: Body weight: 60 kg, Respiratory ventilation: 1 m3/h, Periodic cleaning: 4 times/yr, Working days: 250 d/yr, Protection factor: 1000. *2: See Table 3. *3: National Council for Environment.
Discussion
Estimated dioxin exposure concentration
The volumes of the incinerated waste ranged from 30 to 1,800 tonnes/d with a median of 240 tonnes/d for continuously burning incineration plants of Japan[18]. Because the volumes in the studied three plants were 160, 300 and 600 tonnes/d, these were representatives of little, middle and large plants. The dioxin concentrations in the exhaust gas ranged from 0.00 to 120 ng TEQ/Nm3 with a median of 3.1 ng TEQ/Nm3 for continuously burning incineration plants of Japan (1996-97)[19]. Because the concentrations in plants A, B and C were 8.9-42, 0.82-1.4 and 3.1-12 ng TEQ/Nm3, respectively, these were representatives of high, low and middle levels. Consequently, these plants represent continuously burning incineration plants of Japan.
This study found that the total dust exposure concentrations in incinerator workers were 0.11 to 1.50 mg/m3 in the daily operation. The Committee for Investigation on Prevention of Adverse Health Effects of Chemical Substances in Waste Treatment[20], Japan, reported that the total dust concentrations in the working environment were 0.03 to 0.27 mg/m3 at a general waste incineration plant. The dust level was higher in our study probably because of differences in incineration plant and sampling strategy (exposure vs. working environment). The estimated dioxin exposure concentrations were 0.5 to 7.2 pg TEQ/m3, which were slightly higher than the dioxin concentrations in general air in Japan (0.00 to 1.76 pg TEQ/m3)[21]. The above Committee study found that the dioxin concentrations in the working environment were 0.01 to 0.58 pg TEQ/m3. The dioxin level in our study was about 10 times as high as that in the Committee study. This discrepancy is not unreasonable, because dioxin concentrations in fly ash and slag differ greatly among the incineration plants.
The Ministry of Labor, Japan, recommends an administrative level of 2.5 pg TEQ/m3 for dioxins[22]. However, the above estimation of dioxin exposure concentration suggests that, in the specific cases, the dioxin exposure levels in the incinerator workers exceed the administrative level even in the daily operation, and thus dust emission must be reduced in the workplace and/ or respiratory protective device and protective clothing should be worn.
The total dust exposure concentrations during periodic maintenance were 12 to 2,000 mg/m3, showing high dust levels. The estimated dioxin exposure concentrations during cleaning of the inside of the EP were 370 to 92,000 pg TEQ/m3, which were 150 to 37,000 times as high as the administrative level. The estimated dioxin exposure concentrations during cleaning the inside of the incinerator were 0.5 to 48 pg TEQ/m3, which were considerably lower than in the case of EP, because the dioxin level in slag was much lower than in fly ash. However, the dioxin exposure level in the incinerator also exceeded 2.5 pg TEQ/m3 in most of the cases. Consequently, wearing a respiratory protective device such as an airline mask as well as reducing dioxin levels in fly ash and slag are essential to reduce dioxin intake during periodic maintenance.
Comparison between the two methods of periodic maintenance showed that the total dust exposure level was much higher in method II. This was because the workers removed all of slag as well as clinker in the incinerator and removed all of fly ash by compressed air in the EP. The periodic maintenance was done according to the instructions of the incinerator manufacturer. Considering the very high dust exposure level, these instructions should be revised.
In this study, we observed that because the workers had to enter the EP from a narrow opening and clean it in a lying posture during periodic maintenance, they inhaled scattered fly ash. Consequently, the manufacturer should design the incinerator and EP to not only reduce dioxin synthesis but also to minimize dioxin exposure of incinerator workers during the periodic maintenance. After cleaning the equipment, the workers removed dust adhering to their protective clothing using compressed air and were again exposed to scattered dust. We would recommend using a vacuum cleaner to remove dust from the clothing.
Estimated dioxin intake
In a report of the Ministry of Welfare[19], dioxin concentrations in fly ash ranged from 0.01 to 100 ng TEQ/ g for the continuously burning incineration plants to which the old dioxin guideline was not applicable, and ranged from 0.00 to 8.7 ng TEQ/g for the plants to which the guideline was applicable. These values were obtained by analysis without particle size selection. As stated in the methods, fine particles are potentially inhalable, because they can be easily scattered by disturbance and remain suspended in the air for a long time, while coarse particles cannot. Chang et al.[23] found that fine particles of fly ash had higher dioxin contents than large particles. Consequently, if the dioxin concentration in the fly ash is measured with particle size selection, the highest value for the continuously burning incineration plants must be more than 100 ng TEQ/g. Accordingly, dioxin intake was estimated for the four cases of fly ash of 1, 10, 100 and 1,000 ng TEQ/g.
Dioxin concentrations in slag and in deposited dust were assumed to be 1/100 and 1/10 times, respectively, as high as that in the fly ash. At the three plants, the ratios of dioxin concentration in slag to that in fly ash were 0.0001 to 0.04 with a median of 0.02 (Table 2). In the above report of the Ministry of Welfare[19], the mean of dioxin concentration in fly ash and that in slag were 12 and 0.16 ng TEQ/g, respectively, for plants to which the old dioxin guideline was not applicable, and they were 1.5 and 0.023 ng TEQ/g, respectively, for the plants to which the guideline was applicable. Thus, the above assumption on dioxin concentration in slag is appropriate. At the three plants, the ratios of dioxin concentration in deposited dust to that in fly ash were 0.02 to 0.66 with a median of 0.10 (Table 2). The number of data is too small to regard the median value as a representative one but the value was used in this study, because there were no other reports of these ratios. Actually, the ratio varies among incineration plants. For example, because BF generates less dioxins than EP[19], the ratio of slag to fly ash must be higher in BF than in EP. But if various situations are considered, the number of cases is too large, so a simple assumption was used in this study.
The two methods of periodic maintenance were assumed in the estimation of dioxin intake, because both were observed in the three plants. The frequency of the periodic maintenance was assumed to be 4 times/yr for each worker, because the incineration plants had three or four incinerators, the periodic maintenance of each incinerator was performed every three to four months on a rotation basis, and the workers did it in rotation. However, the frequency and method may differ in other plants, such as 8-h or 16-h burning types. Accordingly, the estimated dioxin intake in this study is only for plants equipped with continuously burning and stoker incinerators.
In 1998, the Ministry of Labor[22] issued a regulative instruction that airline mask or hose mask should be used while working inside an incinerator, EP and BF. When this study was conducted, airline masks were used in plant B, but dust masks were used in plants A and C. At the present time, airline masks have been introduced at the latter two plants, but dust masks are also being used because the number of the airline masks is not enough. Probably, there may be other incineration plants in which dust masks are still used for working inside the incinerator and EP. Consequently, both the uses of dusk mask and airline mask were assumed in the estimation of dioxin intake. In 1970s, the incinerator workers had used gauze masks while working inside the incinerator in the studied plants. Thus, in order to emphasize importance of the respiratory protection, periodic maintenance without the use of a protective device was also assumed.
This study shows that in the case of fly ash of 1,000 ng TEQ/g, even if an airline mask is used during periodic maintenance, the dioxin intake is predicted to exceed the TDI. It is the reason that the intake during the daily operation is predicted to be high. In this case, it is essential to decrease the dioxin concentration in fly ash. In the cases of fly ash of 100 ng TEQ/g, the dioxin intake was predicted to exceed the TDI if the worker does not wear either an airline mask or a dust mask for method I, or if the worker does not wear an airline mask for method II. In the cases of fly ash of 10 ng TEQ/g, it was predicted to exceed the TDI if the worker does not wear either a dust mask or an airline mask for method II. Consequently, when the dioxin concentration in fly ash is not higher than 100 ng TEQ/g, wearing an airline mask during the periodic maintenance would be effective.
Because a fly ash sample is generally analyzed without particle size selection, the measured dioxin concentration may be lower than that in fly ash inhaled by the incinerator worker during the periodic maintenance. Consequently, if the dioxin concentration measured without particle size selection is used to estimate dioxin intake, underestimation would result.
In the three plants, the workers wore protective gloves and clothing while working in the incinerators and EPs. If workers do not wear these protections, because hand to mouth ingestion and dermal absorption could occur, the dioxin intake must be higher than our estimation. This dioxin intake estimation included the daily operation and periodic maintenance but not repair work of the incinerator and EP, which was done by outside workers at all three plants. The dioxin intake by these workers must be examined in the future.
Conclusion
In the daily operation, the dioxin exposure concentrations were estimated to exceed the administrative level in some cases, so dust emission should be reduced in the workplace and/or respiratory protection devices should be used. During the periodic maintenance, because dioxin exposure concentrations were estimated to be very high, wearing high grade respiratory protection devices such as an airline mask is essential for reducing dioxin intake. In cleaning the inside of incinerators and EPs by method II, the estimated values were 150 to 37,000 times as high as the administrative level. Consequently, the cleaning method should also be thoroughly examined when designing the incinerator and EP.
When the dioxin concentration in fly ash is 1,000 ng TEQ/g, even if an airline mask is used during the periodic maintenance, the total of occupational and environmental dioxin intakes by an incinerator worker is predicted to exceed the TDI. Consequently, it is essential to decrease the dioxin concentration in fly ash. When the dioxin concentration in fly ash is no higher than 100 ng TEQ/g, if an airline mask is used during the periodic maintenance, the dioxin intake is predicted to be lower than the TDI. In this instance, wearing an airline mask would be effective at lowering dioxin intake below the TDI.
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