Occupational Inhalation Health Risk Assessment of TCE Exposure in the Korean Manufacturing Industry

Abstract

This study aimed to assess the health risks to workers in the Korean manufacturing industries using trichloroethylene (TCE). In Republic of Korea, TCE has permissible exposure limits. In 2014, the permissible exposure limits were exceeded in two cases, necessitating a health risk assessment. The exposure value of TCE in the workplace was determined using Korea’s Workplace Environmental Monitoring Program (WEMP), and risk assessment was performed by applying a probabilistic distribution using a Monte Carlo simulation. When evaluating non-carcinogenic risks at the 50th percentile, all 20 industries had acceptable risk, and at the 95th percentile, 12 of the 20 industries had unacceptable risk. Following the cancer risk assessment, 17 out of 20 industries had unacceptable risks, and the 95th percentile of all industries had unacceptable risks. The non-carcinogenic and carcinogenic risks of TCE were highest during metal fabrication. Based on these results, metal fabrication was chosen as an industry that should receive management priority in Korea. The findings of this study serve as a foundation for managing TCE among manufacturing workers.

1. Introduction

Trichloroethylene (TCE) is a non-flammable, volatile, colorless, sweet-smelling organic solvent. In the workplace, 80–95% is used for degreasing and cleaning metal machine parts [1,2]. In Republic of Korea (referred to as Korea), TCE is applied in automotive and engine parts, electrical materials, computers, radio-televisions, communication equipment, metal products, glass and optical instrument cleaning, various cleaning activities, and degreasing of rubber and leather products [1,3,4,5]. In 2019, 515 manufacturing establishments with 5 or more employees handled TCE, totaling 7236 tons [6].

TCE diffuses primarily as vapor; therefore, respiratory exposure is the primary route, and skin absorption is limited under normal working conditions [7]. TCE vapors flow through the bloodstream and affect the central nervous system, causing headaches, dizziness, vomiting, drowsiness, and in severe cases, loss of consciousness or death [8,9,10,11,12,13,14]. Long-term exposure can cause nervous system symptoms, such as memory loss, loss of motivation, emotional instability, and permanent damage to the liver or kidneys [8,9,10,11,12,13,14]. In addition, there have been several case reports indicating that severe systemic dermatitis may occur. This typically manifests as generalized exfoliative dermatitis, such as Stevens–Johnson syndrome or toxic epidermal necrolysis. Case reports have indicated that toxic hepatitis is frequently concurrent with this type of generalized exfoliative dermatitis [15]. High concentrations of trichloroethylene fumes cause deaths [8]. There have been cases of deaths resulting from inhalation of abnormally high levels of TCE vapor in the workplace [8]. TCE is a human carcinogen [8,9,10,11,12,13]. Strong evidence for kidney cancer is found in the existing human data [8]. TCE is described as a group 1 carcinogen by the International Agency for Research on Cancer (IARC) and a group A2 carcinogen by the American Conference of Governmental Industrial Hygienists (ACGIH) [8,16]. There is sufficient evidence that TCE causes kidney cancer in humans [9,17].

Several independent studies have observed an effect on liver cancer due to occupational exposure to TCE. In 9 of 15 studies, in which occupational TCE exposure was identified in individual subjects, the relative risk (RR) ranged from 0.93 to 1.39 for overall exposure to TCE [18,19,20,21,22,23,24,25,26], and in 8 studies, in which exposure to TCE was more likely than the overall population, the RR ranged from 1.04 to 1.79 [18,19,20,21,22,23,24,25]. Additionally, the most estimated RR in 15 of the 24 studies reporting kidney cancer risk was between 1.10 and 1.90 for overall exposure to TCE [27,28,29,30]. There are also risk assessment studies on TCE used in industry [31,32,33], but they are insufficient compared to epidemiological studies.

In January 2006 in Korea, a male worker performing TCE cleaning in the electronics parts manufacturing industry died because of Stevens–Johnson syndrome, an acute form of TCE poisoning. Around the same time, a woman who worked in the inspection and packaging process following TCE cleaning in the mobile phone parts manufacturing industry developed Stevens–Johnson syndrome and died [1]. In 2008, three workers were hospitalized for toxic hepatitis [34]. Despite the numerous health risks mentioned above, TCE is widely used in the workplace because of its excellent solubility in oils, fats, and resins, volatility, non-flammability, and low cost [14].

In Korea, the occupational exposure limits (OEL) of TCE are defined as a Threshold Limit Value–Time-Weighted Average (TLV-TWA) of 50 ppm, and a Threshold Limit Value Short-Term Exposure Limit (TLV-STEL) of 200 ppm [35,36]. These were subsequently revised on 17 February 2016 and are currently TLV-TWA 10 ppm and TLV-STEL 25 ppm, as proposed by the ACGIH [35,36].

In accordance with the Occupational Safety and Health Act, a designated measurement institution measures 190 types of harmful factors in the workplace and reports the results to the Minister of Employment and Labor to ensure that they are managed below the regulatory threshold. This is known as the Workplace Environmental Monitoring Program (WEMP) [35,37]. The WEMP data include information such as industry type, main products, department name, process code, process name, workplace unit, name of the measured substance, measurement time, and exposure concentrations of the substances, reported as TWA [37,38]. The risk of TCE was recognized as a substance that caused many occupational diseases in 2006. Thus, TCE has been designated as a permissible substance since 2009. An exposure standard has been established at a permissible level, making it legally mandatory for employers to maintain a work environment below the permissible standard [35,37].

Therefore, TCE has caused many occupational diseases in workers in Korea, and TCE is a permissible substance that must not exceed a single permissible level in a working environment. However, in the 2014 WEMP, two cases (two workers) exceeded the permissible level, demonstrating the urgent need to assess workers’ health risks. Thus, in this study, we evaluated TCE exposure levels and health risks using WEMP and ranked industries that require immediate management. Furthermore, we aimed to confirm the appropriateness of TCE institutional management at the time of measurement and suggest methods to reduce TCE-related health risks, thereby providing basic data for the management of workplaces using TCE in Korea.

2. Materials and Methods

Figure 1 presents a diagram of methods. In total, 7046 TCE measurements were obtained from WEMP data. The industry classified the measured values using the Korean Standard Industrial Classification (KSIC 10th) [39]. Only industries with more than 30 measured values were chosen as research subjects to assume a log-normal distribution [40,41]. Excluding STEL measurements, this study used 6311 measurements in total. Of the 6311 classified data points for the 20 industries, 2923 measured values were 0. To avoid statistical underestimation, a measurement value of 0 was corrected to the detection limit/2 when calculating the mean (AM) and standard deviation (SD) [42]. The detection limit of TCE was determined by referring to the technical guidelines of the Korea Occupational Safety and Health Agency (KOSHA) to measure and analyze the work environment for TCE [43].

The risk assessment of TCE was conducted according to the Korea Occupational Safety and Health Agency’s Hazard and Risk Assessment Guidelines for Chemical Substances (KOSHA GUIDE W-6-2021) [44] based on the Environmental Protection Agency (EPA) model and the Occupational Safety and Health Research Institute’s (OSHRI) Hazard and Risk Assessment Research Report on Hazardous Chemical Substances [45]. Because inhalation is workers’ primary mode of TCE exposure, this study conducted carcinogenic and non-carcinogenic assessments for inhalation. For the Point of Departure (POD), the No Observed Adverse Effect Level (NOAEL) and Inhalation Unit Risk (IUR) values were obtained from the Environmental Protection Agency Integrated Risk Information System (EPA IRIS). In 1999, EPA applied the concept of nonlinear extrapolation to liver cancer caused by TCE [46]. According to an EPA report, using nonlinear extrapolation, the risk assessment method depends on the procedure used. If an RfD or RfC was calculated, the hazard can be expressed as a hazard quotient (HQ) [47,48].

The IUR was converted to reference concentrations in workplaces of carcinogenicity (RfCwork_carcinogen) based on a risk level of 10⁻⁴ [45,49]. The IUR is the risk that a 70 kg adult would be exposed to 24 h a day for 70 years, and conversion is required to apply it to workplace risk assessment. Equation (1) is the process of converting the IUR to the workplace risk value, and Equation (3) is the process of converting Rfcwork_carcinogen, which is the concentration at risk level 10⁻⁴, which is the permitted concentration in the workplace [45,49]:

IURworkplace = IUR ÷ CF

(1)

CF = AsRV/KwRV × AEF/KwEF × AED/KwED × ABW/KwBW

(2)

Here, AsRV is the American standard Respiratory Volume, KwRV is the Korean worker Respiratory Volume, AEF is the American Exposure Frequency (days/year), and KwEF is the American and Korean worker Exposure Frequencies (days/year). AED is the American Exposure Duration (years), and KwED is the Korean worker Exposure Duration (years). ABW is the American body weight (kg), and KwBW is the Korean worker body weight (kg) [45,49].

RfCwork_carcinogen = 1/(IURworkplace (risk/ppm)) × 10⁻⁴ (risk)

(3)

The RfCwork for carcinogenicity and non-carcinogenicity applied appropriate coefficients to Korean workers, and correction coefficients were applied based on a KOSHA research report [45].

HQ = EC (ppm) ÷ RfCwork_{non-carcinogen} (ppm)

(4)

HQcarcinogen = EC (ppm) ÷ RfCwork_carcinogen (ppm)

(5)

Here, the exposure concentration (EC), the concentration of TCE, is a value recorded in the WEMP by measuring TCE in personal samples from all Korean workplaces that handle TCE and analyzing them using gas chromatography [43].

The hazard quotient (HQ) was used to determine the non-carcinogenic and carcinogenic risks to workers. The HQ was obtained by dividing the TCE concentration by RfCwork_{non-carcinogen} and RfCwork_carcinogen according to Equations (4) and (5). If HQ exceeds 1, it represents a risk that workers cannot accept; if HQ is less than 1, it represents an acceptable risk to workers [31,38].

Using R program (ver. 4.4.0, R foundation, Indianapolis, IN, USA), the Monte Carlo simulation was repeated 10,000 times, and the risk assessment was performed by applying a probabilistic distribution [50,51]. The HQ was derived by assuming a log-normal distribution for the TCE concentration, a measured value for workers.

In addition, the measured values were compared with the Occupational Exposure Limit (OEL) governing workplace exposure levels to confirm the adequacy of the institutional management of TCE at the time of measurement. Among the 20 manufacturing industries, the exposure concentration of the 95th percentile, the worst case for the industry with the highest risk, was compared with the OEL in 2014 and the current OEL.

Therefore, we applied the TCE reduction method according to personal protective equipment to the largest industry with an HQ to determine whether the risk was reduced. A Monte Carlo simulation was performed using open-source R statistics by applying the personal protective equipment wearing rate and mask efficiency of workplaces with 50 or more employees, which account for most of the Korean manufacturing industry.

3. Results

3.1. Dose–Response Assessment

In this study, among the six non-carcinogenic inhalation tests on the kidneys using experimental animals in the EPA toxicological review, the study by Maltoni (1986) was chosen because it met the chronic toxicity period and provided the best description of the experimental species. Furthermore, this study provided key data on repeated dose toxicity by inhalation from the REACH registry dossier of the European Chemicals Agency (ECHA). Therefore, this value was selected as the NOAEL value [52,53].

In a 104-week (5 days/week, 7 h/day) inhalation experiment using Sprague-Dawley rats, renal tubular megakaryocytosis was observed in male rats in the 300 and 600 ppm groups. Therefore, the NOAEL of POD was determined to be 100 ppm. The calculation of RfCwork_{non-carcinogen} was derived using quantitative correction and uncertainty coefficient correction, as shown in

Table 1. Values for the derivation of the RfCwork_{non-carcinogen} of TCE.

Steps	Categories		Values
POD			NOAEL 100 ppm (rat, inhalation)
Quantitative correction	NOAELADJ (7/8 × 5/5 × 0.83/1.25)		0.58
	NOAELHEC		1
Uncertainty correction	Interspecies		3
	Intraspecies		5
	Duration	≥6 months	1
	Severity		1
RfCworknon-carcinogen	100 ppm × 0.58 ÷ 15		3.87 ppm

[44,45].

Through quantitative correction, the NOAEL Adjusted for Duration (NOAELADJ) and the NOAEL Adjusted to a Human-Equivalent Concentration (NOAELHEC) were derived by correcting for 5 days per week, 8 h of daily exposure, and the difference in respiratory rates between workers and the public [38,44,45].

The IUR for TCE presented by the EPA IRIS was used as the carcinogenic toxicity value of TCE for the kidney to calculate the RfCwork_carcinogen for inhalation, as shown in

Table 2. Values for the derivation of the RfCwork_carcinogen of TCE.

Steps	Categories	Values
POD	Kidney	Inhalation unit risk 5.49 × 10−3 risk/ppm
IURworkplace 1	IUR ÷ CF = IUR ÷ 4.2(20/10 × 365/260 × 70/40)/(70/60)	1.31 × 10−3 risk/ppm
RfCworkcarcinogen 2	1 ÷ (1.31 × 10−3 risk/ppm) × 10−4 risk	7.65 × 10−2 ppm

IURworkplace ¹, inhalation unit risk in the workplace; RfCwork_carcinogen ², reference concentration of carcinogenicity in the workplace.

[54].

3.2. Exposure Assessment

The AM and SD values of the TCE measurements are listed in

Table 3. Arithmetic mean and standard deviation for the measurement values.

KSIC Code	Type of Manufacturing Industry	AM (ppm)	SD	N	Range (ppm)
25	Fabricated metal products (except machinery and furniture)	4.67	7.94	1175	0~86.48
27	Medical, precision, and optical instruments, watches, and clocks	4.07	6.88	235	0~33.53
26	Electronic components, computers, visual, sounding, and communication equipment	3.62	6.71	1346	0~46.72
33	Other	3.55	7.42	130	0~49.29
29	Other machinery and equipment	2.89	6.19	618	0~48.80
28	Electrical equipment	2.53	6.72	262	0~57.28
30	Motor vehicles, trailers, and semitrailers	2.09	4.32	623	0~41.76
16	Wood and products of wood and cork, except furniture	2.07	5.88	71	0~36.19
23	Other non-metallic mineral products	1.62	5.04	39	0~27.21
17	Pulp, paper, and paper products	1.54	6.52	47	0~35.99
24	Basic metals	1.22	3.21	180	0~21.47
22	Rubber and plastic products	1.18	4.36	261	0~38.59
18	Printing and reproduction of recorded media	0.88	3.53	165	0~26.83
20	Chemicals and chemical products (except pharmaceuticals and medicinal chemicals)	0.66	3.23	379	0~32.33
21	Pharmaceuticals, medicinal chemicals, and botanical products	0.42	2.06	70	0~15.58
32	Furniture	0.38	0.80	36	0~4.08
15	Leather, luggage, and footwear	0.24	0.32	41	0~1.15
13	Textiles, except apparel	0.21	0.80	124	0~8.18
31	Other transport equipment	0.13	0.92	461	0~12.68
19	Coke, briquettes, and refined petroleum products	0.02	0.10	48	0~0.55

. The five industries with the largest AM presence were manufacturers of fabricated metal products (4.67 ppm), manufacturers of medical, precision, and optical instruments, watches, and clocks (4.07 ppm), manufacturers of electronic components, computers, visual, sounding, and communication equipment (3.62 ppm), other manufacturing (3.55 ppm), and manufacturers of other machinery and equipment (2.89 ppm). The industry with the lowest AM was coke, briquettes, and refined petroleum products’ manufacturers (0.02 ppm).

3.3. Risk Characterization

Table 4. Non-carcinogenic HQ results for trichloroethylene.

KSIC Code	25%ile	50%ile	75%ile	95%ile	HQ > 1 (%)
25	2.8 × 10−1	6.1 × 10−1	1.4	4.0	33.5
27	2.5 × 10−1	5.4 × 10−1	1.2	3.6	30.5
26	2.0 × 10−1	4.5 × 10−1	1.0	3.3	25.0
33	1.6 × 10−1	4.0 × 10−1	9.7 × 10−1	3.3	23.8
29	1.3 × 10−1	3.2 × 10−1	7.7 × 10−1	2.7	18.6
30	9.9 × 10−2	2.5 × 10−1	5.8 × 10−1	2.0	13.1
28	8.8 × 10−2	2.3 × 10−1	6.1 × 10−1	2.4	15.0
16	6.4 × 10−2	1.8 × 10−1	4.7 × 10−1	2.1	12.5
23	4.5 × 10−2	1.2 × 10−1	3.7 × 10−1	1.5	9.0
24	4.2 × 10−2	1.1 × 10−1	3.0 × 10−1	1.2	6.5
17	2.9 × 10−2	9.2 × 10−2	2.8 × 10−1	1.5	7.8
22	2.5 × 10−2	7.7 × 10−2	2.4 × 10−1	1.2	6.0
18	1.7 × 10−2	5.3 × 10−2	1.7 × 10−1	8.5 × 10−1	4.1
32	1.8 × 10−2	4.2 × 10−2	1.0 × 10−1	3.6 × 10−1	0.8
15	1.8 × 10−2	3.6 × 10−2	7.3 × 10−2	2.0 × 10−1	0.1
20	1.0 × 10−2	3.3 × 10−2	1.1 × 10−1	6.8 × 10−1	2.9
21	6.1 × 10−3	2.1 × 10−2	7.1 × 10−2	4.2 × 10−1	1.6
13	4.8 × 10−3	1.5 × 10−2	4.4 × 10−2	2.2 × 10−1	0.5
31	1.4 × 10−3	5.0 × 10−3	1.8 × 10−2	1.2 × 10−1	0.3
19	5.4 × 10−4	1.6 × 10−3	4.9 × 10−3	2.3 × 10−2	0.0

and

Table 5. Carcinogenic HQ results for trichloroethylene.

KSIC Code	25%ile	50%ile	75%ile	95%ile	HQ > 1 (%)
25	13.6	30.8	67.4	217	99.8
27	12.1	26.3	57.5	177	99.9
26	9.8	22.8	51.5	172	99.5
33	8.4	20.5	49.0	172	99.1
29	6.8	16.2	38.3	134	98.3
30	5.1	12.2	29.0	103	97.3
28	4.6	12.1	31.8	126	95.5
16	3.3	9.2	24.9	101	92.9
23	2.3	6.4	18.2	83.8	88.8
24	2.1	5.6	14.9	59.2	88.7
17	1.5	4.6	15.0	74.3	81.9
22	1.3	3.9	11.8	60.2	80.1
18	8.7 × 10−1	2.7	8.6	45.7	72.6
32	8.6 × 10−1	2.1	5.1	17.3	71.7
15	9.2 × 10−1	1.9	3.6	9.8	72.4
20	5.0 × 10−1	1.7	5.7	34.4	61.9
21	3.2 × 10−1	1.1	3.8	19.9	50.9
13	2.4 × 10−1	7.3 × 10−1	2.3	10.7	42.0
31	6.7 × 10−2	2.5 × 10−1	9.3 × 10−1	6.1	24.2
19	2.7 × 10−2	8.1 × 10−2	2.5 × 10−1	1.3	63.9

show the results of the Monte Carlo simulation for non-carcinogenic HQ and carcinogen HQ by industry. The 50th percentile HQ was used to represent the at-large workplace exposure by industry, whereas the 95th percentile HQ was assumed to represent the worst exposure.

3.3.1. Non-Carcinogenic Risk

The HQ of all 20 industries in the 50th percentile was less than 1. However, at the 95th percentile of the HQ, 12 of the 20 industries had an HQ greater than 1. In Figure 2, the 50th percentile, which corresponds to overall workplace exposure, shows the top five manufacturing industries with high HQ as fabricated metal products (0.61); medical, precision, and optical instruments, watches, and clocks (0.54); electronic components, computer, visual, sounding, and communication equipment (0.45); other manufacturing (0.40), and other machinery and equipment (0.32), and the lowest industry as coke, briquettes, and refined petroleum products (0.0016). Based on the 95th percentile, the top five industries with the highest and lowest HQs were the same. Therefore, the HQ of the 50th percentile of all manufacturing industries did not exceed 1, indicating that the overall TCE exposure of workers was at an acceptable level for non-carcinogenic risks. However, in 60% of the 20 manufacturing industries, the HQ of the 95th percentile exceeded 1, indicating that the worst exposure in some manufacturing industries that handle TCE was at an unacceptable level for non-carcinogenic risk.

3.3.2. Carcinogenic Risk

According to the cancer risk assessment, 17 of 20 industries in the 50th percentile HQ exceeded 1, whereas all industries in the 95th percentile HQ performed well and exceeded 1. In Figure 3, the top five manufacturing industries with high HQ at the 50th percentile HQ are shown as fabricated metal products (30.7); medical, precision, and optical instruments, watches, and clocks (26.3); electronic components, computer, visual, sounding, and communication equipment (22.8); other manufacturing (20.5), and other machinery and equipment (16.2), and the lowest industry was coke, briquettes, and refined petroleum products (0.081). Based on the 95th percentile, the top five industries with the highest and lowest HQs were identical. The exceedance rates of the 50th and 95th percentile HQ of the 20 manufacturing industries indicated unacceptable risks of 85% and 100%, respectively.

3.3.3. Comparison of OEL and High-Concentration Exposure

To confirm the appropriateness of the institutional management level, the exposure values from 2014, when the measurements were taken, were compared with the exposure standards. The exposure concentration was chosen from 20 manufacturing industries by taking the 95th percentile of the fabricated metal products’ manufacturing industry, excluding machinery and furniture, which have the highest risks of carcinogens and non-carcinogens. The worst-case exposure concentrations for non-carcinogenic and carcinogenic cases were 15.60 ppm and 16.70 ppm, respectively. This is below the TCE OEL of 50 ppm in 2014 but exceeds Korea’s TCE OEL of 10 ppm, revised in 2016.

3.3.4. Risk Reduction Scenario

Among the non-carcinogenic risks, 60% of the 95th percentile HQ exceeded the acceptable risk standard of 1; among the carcinogenic risks, 85% of the 50th percentile HQ exceeded 1. In the 95th percentile HQ, all industries exceeded 1. Because TCE is a VOC, most reduction methods considered in this study were tested for VOCs. The reduction method was applied to manufacturing fabricated metal products with the highest non-carcinogenic and carcinogenic HQ values. As local exhaust ventilation is mandatory in Korea, it was not included in the reduction method. The proportion of workplaces with fewer than 50 employees among Korean manufacturing companies was 98.7% [55], and among these companies, the personal protective equipment wearing rate of 32.7% to 44.3% was applied [56], and the efficiency of respirators was also applied. Through a Monte Carlo simulation, the 50th percentile HQ and 95th percentile HQ for carcinogenicity and non-carcinogenicity were re-derived, and the reduction ratio of the rate at which the HQ exceeded 1 was derived.

The VOC reduction efficiency using half-type masks with an activated carbon filter was 26%, and that using a half-type respirator mask with an activated carbon filter was 83% [57]. The reduction efficiency of a half-type respirator mask with cartridges replaced once a day was 83.6% [58], and that of a half-type respirator mask with cartridges replaced once a month was 46.6% [58].

Consequently,

Table 6. Scenario of risk reduction.

Factor	Efficiency	Non-Carcinogenic Risks			Carcinogenic Risks
		HQ for 50%ile	HQ for 95%ile	Ratio of HQ > 1 *	HQ for 50%ile	HQ for 95%ile	Ratio of HQ > 1 *
Air-purifying respirators (active carbon filter cartridge)	83%	3.3 × 10−1	3.4	1.56	16.5	147.8	1.03
Half face masks (active carbon filters)	26%	5.4 × 10−1	3.9	1.12	27.2	192.9	1.00
Half mask respirator (twice-a-day cartridge exchange)	83.6%	3.2 × 10−1	3.1	1.60	15.9	158.5	1.03
Half mask respirator (once-a-month cartridge exchange)	46.6%	4.7 × 10−1	3.4	1.22	24.8	179.5	1.00

* Ratio of HQ > 1 (%) in the fabricated metal products industry and HQ > 1 (%) in the fabricated metal products industry with reduced TCE concentration.

shows that the non-carcinogenic risk reduction effect decreased by 1.56 and 1.6 times in the ratio of HQ exceeding 1 when the mask efficiency was 83% and 83.6%, decreased by 1.22 times when the mask efficiency was 46.6%, and when the mask efficiency was 26%, it decreased by 1.12 times. However, the reduction effect of the ratio of HQ of 1 or more in cancer risk was reduced by 1.03 times when wearing masks with 83% and 83.6% efficiency, and there was no reduction effect by 1.00 times when wearing masks with 26% and 46.6% efficiency.

4. Discussion

Workers involved in various processes that use TCE, especially degreasing processes, may be exposed to higher levels of TCE than the general population. High levels of TCE have been measured in environments where TCE is used as an industrial solvent. The metal industry traditionally used TCE for degreasing in the 1970s, and metal cleaning accounted for approximately 65% of the total metal processing in the USA [59]. In this study, the manufacturing of fabricated metal products was found to be the industry with the highest AM (4.67) and the industry with the second-largest number of measurements (1175) in Korea.

Exposure to TCE as a metal cleaner or degreaser is prevalent in Korea, as well as in other countries. The average TCE concentration of 30 dry cleaners in Iran was much higher than that of the non-occupationally exposed group [60]. In Denmark, 69% of the measurements from 150 companies between 1947 and 1989 were in the steel and metal industry, indicating higher concentrations than in all other industries [61]. In a watch manufacturing factory in Thailand, 171 workers used TCE to clean the surfaces of the metal parts of watches [62]. The average exposure to TCE in the workplace air was 27.83 ± 6.02 ppm [62]. In the United States, AM, including both individual and area measurements, was 39.6 ppm (n = 867) of trichloroethylene exposure in degreasing operations [59]. In a study by Melissa et al., when measuring the TCE concentration by occupation in Shanghai, China, the air concentration of metal processing workers using TCE as a degreasing agent was found to be exposed to GM at levels of 29–34 mg/m³ [63]. The TCE exposure level at several electroplating and electronics facilities in China was 58.63 mg/m³ for TCE in the degreasing group, and 58.63 mg/m³ for TCE in the washing group [31]. These results are consistent with the fact that TCE use is decreasing in the United States and Europe due to its toxicity but is increasing in some Asian countries due to increasing demand for degreasing agents [3].

TCE use in China increased from 4 to 5 kilotons in 1980–1990 to 40 kilotons in 2000 and 165 kilotons in 2010, of which 60–75% was used for degreasing [3]. Additionally, a study from China analyzed 212 workers and found that the C-TWA exceedance rate for TCE was 80.7% [64]. Therefore, it can be concluded that such high levels of TCE exposure are due to the country’s usage and the industries where it is used.

According to the non-carcinogenic risk assessment, the 95th percentile of HQ indicates high-concentration exposure, with 60% of the 20 manufacturing industries showing an unacceptable risk. The results of the carcinogenic risk assessment in this study showed that the HQ exceeded 1 in all industries at the 95th percentile, indicating exposure to high concentrations. Additionally, 85% of industries at the 50th percentile HQ, corresponding to average exposure, showed unacceptable risk.

In China, 14 electroplating companies and 6 electronics companies conducted risk assessments for non-carcinogenic and carcinogenic risks for workers who used TCE for degreasing work [31]. The HQ for non-carcinogenic risks was 18,000, while the HQ for workers involved in cleaning work was 33,000 [31]. The unacceptable risk ratio for carcinogenesis in the degreasing group was 55%, while in the cleaning group, it was 50% [31]. A European study found that the average concentration of TCE in a metal degreasing workshop exceeded the MAK value, indicating an unacceptable risk with a hazard index of 1.68 [33]. In addition, a health risk assessment was conducted on 25 workers exposed to TCE in a computer cleaning department in Thailand. The HQ for non-carcinogenic effects among workers in the cleaning parts was 845, indicating an unacceptable risk [32]. The excess lifetime cancer risk for carcinogenicity is 0.162, indicating an unacceptable risk [32].

Regarding epidemiologic studies, 24 studies were included, such as a European study on the incidence of cancer in iron and metal industry degreasers using TCE and a US study on mortality in electrical, aerospace, and aircraft maintenance workers using TCE [65]. The incidences of kidney cancer, liver cancer, and non-Hodgkin’s lymphoma among workers exposed to TCE were 1.27 times higher, 1.29 times higher, and 1.23 times higher, respectively [65]. According to Moore et al., the risk of kidney cancer increases with TCE use for more than 13.5 years [30]. TCE was first recognized as an occupational cause of renal cell carcinoma in Korea in 2013, with cases of occupational death due to TCE in the workplace [66].

This study indicated that workers in the fabricated metal products industry are likely to be exposed to higher levels of TCE than workers in other industries. However, for workers exposed to TCE, the process is also a major factor in assessing risk. This aspect was not covered in this study, indicating the need for further research. Additionally, the literature review found that non-carcinogenic and cancer risk values from exposure to TCE in several countries exceeded acceptable levels, emphasizing the need for further action. However, the RfC standard values used to calculate risk in each country vary, which may lead to underestimation or overestimation of risk results.

In this study, the exposure level in the manufacturing of fabricated metal products, the industry with the highest risk, was compared with the OEL in 2014, the measurement year of the WEMP, and the current OEL in Korea. Liver damage, accompanied by systemic skin diseases caused by TCE, has an immunological mechanism that can occur even at low concentrations. Internationally, cases of Stevens–Johnson syndrome due to exposure to TCE have been reported to occur at exposure levels below 50 ppm or, in a small number of workers, at exposure levels of less than 25 ppm [35,67]. Measurements of three small cleaning workshops in Korea where Stevens–Johnson syndrome occurred revealed levels of 32.3 ppm and 21.97 ppm in Workplace 1, 30.08 ppm in Workplace 2, and 107.2 ppm in Workplace 3 [15]. TCE has been found to cause occupational diseases even at exposure levels lower than 50 ppm, Korea’s permissible standard in 2014 [15]. Therefore, revising the 2016 TCE exposure standard from 50 ppm to 10 ppm was reasonable. However, as workplaces that handle TCE are managed in accordance with the revised OEL, additional evaluation of workers’ TCE exposure is necessary after the OEL is revised to 10 ppm.

The risk assessment and characterization results of this study indicated that exposure management of TCE was the most urgent requirement in the manufacturing of fabricated metal products. Therefore, to utilize it in policy research for TCE management in the workplace, reduction methods have been applied to manufacturing fabricated metal products to investigate the reduction effect of TCE. As a result, the best strategy to reduce risk was wearing an activated carbon electric air-purifying respirator or a gas mask with cartridge replacement every two days. Respirators are a variable tool, but their actual reduction effect is minimal, so using respirators to protect workers from TCE is insufficient in managing carcinogenic risk. Therefore, better management measures, such as engineering measures or substitutes for chemicals, must be considered.

The working conditions of each workplace could not be determined from WEMP data and may be less representative because they were measured once each in the first and second halves of the year. Therefore, more data are required on the specific work environments of individual workplaces. Additionally, the WEMP used in this study evaluated human exposure only through respiratory exposure and did not consider dermal exposure. However, dermal exposure to TCE may occur during the cleaning and degreasing processes. Therefore, further studies on the effects of dermal exposure are required.

This is the first TCE risk assessment study using national data from Korean manufacturing plant workers. This study suggests that even if workers’ average exposure concentration to TCE is below the OEL, workers’ exposure levels may become hazardous when the exposure levels are higher than the RfC. Therefore, this study not only provides significant data for workplaces handling TCE in Korea but also provides data that are useful in managing health risks due to TCE exposure in future workplaces.

5. Conclusions

The purpose of this study was to identify the industries that should be prioritized for risk management of Korean workers exposed to TCE. This study used WEMP data to calculate the HQ for non-carcinogens and carcinogens to assess the risk in manufacturing industries using TCE. The non-carcinogenic and carcinogenic risks of TCE were the highest in the manufacturing of fabricated metals, and the risk was relatively low in the manufacturing of coke, briquettes, and refined petroleum products. In addition, 33.5% (non-carcinogenic) and 99.8% (carcinogenic) of workers in Korea’s fabricated metal manufacturing industries showed exposure levels above the RfC, raising concerns about adverse effects on workers’ health. Therefore, TCE exposure control should be a priority in fabricated metal manufacturing. However, when evaluating the carcinogenic risk in the reduction scenario, the reduction effect of respirators was found to be minimal. Therefore, future studies are needed to evaluate health risks to workers resulting from effective reduction strategies for the carcinogenic risk of TCE.