Assessment of exposure of children swimmers to trihalomethanes in an indoor swimming pool

This study aimed to understand the exposure levels of trihalomethanes (THMs) in an indoor swimming pool and calculate the risks of exposure to THMs, based on the presence of each THM species, of children swimmers aged 6–17, in Beijing, China. We obtained exposure factors for the children through questionnaires and measured THM concentrations through laboratory tests, and we combined the results with an exposure model to calculate the risks, with consideration of different exposure routes (oral ingestion, inhalation and dermal absorption). In terms of exposure factors for the swimmers aged 6–17, the average body weight, exposure duration, exposure frequency, swimming time, shower time, changing time, warm-up exercise and rest time, skin surface area and ingestion rate of pool water were 40.46 kg, 2.70 years, 96 events/year, 64.03 min/event, 17.04 min/event, 15.31 min/event, 12.71 min/event, 1.37 m and 48.93 ml/event, respectively. The THM concentrations in swimming pool water, shower water, swimming pool air and locker room air were 67.17 μg/L, 12.64 μg/L, 358.66 μg/m and 40.98 μg/m, respectively. The average cancer risk of THMs was 5.44 × 10 , which is an unacceptable risk according to the United State Environmental Protection Agency (USEPA) Guidelines. The average hazard index was 0.007, i.e., less than 1, indicating that the noncancer risk was acceptable. Chloroform (TCM) was the main substance in four species of THMs and inhalation exposure was the main exposure pathway. The risk of cancer and noncancer from inhalation exposure to THMs accounts for 97–99% of the total risk. As a result, the disease control authorities and administrative department should pay attention to the health and safety of swimming facilities and, at the same time, establish standards for THMs in the air through further research.


INTRODUCTION
Swimming is praised as the best sport in the 21st century.
With the development of society, swimming has gradually become a popular recreational fitness sport. Duan (Xiaoli ) shows that people prefer swimming in public swimming places, and indoor swimming pools are an important type of public swimming pool. There are many ways to disinfect indoor swimming pools. Chlorination is a common disinfection method for swimming pool water, and it is the most effective and lowest-cost method compared with others (Daizhi et al. ). However, while chlorine inactivates pathogens in swimming pools, it causes reactions with natural organic matter, bromide, iodide and human inputs (e.g., constituents of sweat and urine, skin particles, hair, microorganisms, cosmetics, and other personal care products), forming disinfection byproducts (DBPs) (Hrudey ). Though over 100 different DBPs have been identified thus far, the most commonly detected Health Organization (WHO) points out that indoor swimmers can be exposed to THMs mainly by three ways, including oral ingestion, dermal contact and inhalation pathway. Thus, in this study, THMs can be taken in by child swimmers through direct ingestion of pool water, dermal contact with the water, and inhalation of THMs volatilized into indoor air from chlorinated water.
Studies have showed that exposure to THMs could cause damage to health. Several epidemiologic studies have reported that populations exposed to THMs through ingestion exhibit elevated rates of bladder, colon, and rectal cancers (Hamidin et  requires more experimental evidence. TCM is also listed as a reproductive toxicant. Therefore, people should pay attention to the health risk assessment of THMs in swimming venues. Health risk assessment is divided into carcinogenic and non-carcinogenic risk assessment. Among them, carcinogenic risk refers to the incidence of cancer that exceeds normal levels caused by exposure to carcinogens in a lifetime. The noncarcinogenic chronic toxicity of chemical pollutants in the swimming pool is generally measured relative to the reference dose (RfD). Exposure levels above the RfD may be dangerous. Exposure levels equal to or lower than the RfD are unlikely to be dangerous. As a susceptible population, children are more vulnerable to swimmingrelated health risks. Villanueva et al. () showed that the longer a child swims, the greater the risk of asthma. Bernard et al. () found that the longer the swimming time, the higher the risk of bronchiolitis among 6-year-old children. Pándics et al. () reported that the cancer risk level of children aged 7-10 was 1.98 × 10 À6 and that of children aged 11-14 was 1.19 × 10 À6 in the swimming pool. Paopuree et al. () conducted a health risk assessment of children aged 6-12 years exposed to THMs during swimming, and showed the cancer risk was 1.98 × 10 À4 .
According to previous studies, the predicted health risks were higher than the USEPA regulatory limit of 10 À6 in most cases. Thus, the health risks of children's exposure to THMs in the swimming pool should be a critical concern.
However, in China, many studies focused on adults to do the health risk assessment of THMs exposure, but few studies reported the health risks of children exposed to THMs in swimming pools. Meanwhile, these adult studies only considered exposure to THMs during swimming, but did not consider exposure while changing clothes, showering, doing warm-up exercises and resting. In addition, in these studies the exposure factors adopted in the health risk assessment model mainly refer to the recommendation of the USEPA. While due to differences in geographic regions, diet and living habits, the exposure factors vary widely among countries. So for Chinese people there is a certain bias in the risk assessment based on USEPA exposure factors. Therefore, this study aims to do some preliminary work on the relevant exposure factors of children swimmers and then make the health risk assessment of THMs exposure. A questionnaire survey was carried out to obtain the exposure factors of children in one indoor swimming pool, and THMs concentrations in swimming pool water and in the surrounding air were collected and analyzed. Then the THMs exposure level of children via different pathways were estimated. Finally, cancer risk and noncarcinogenic risk were evaluated.

Questionnaire
A questionnaire survey was conducted through face-to-face interviews at an indoor swimming pool in Beijing, China, from July to August 2019. The survey method can make the questionnaire results more accurate, improve the quality of the questionnaire results, and the survey time can be more flexible. In the face-to-face questionnaire survey, it is required to be in a relatively quiet and comfortable environment as far as possible, and to avoid the presence of unrelated personnel, otherwise the research subject may cause relevant interference factors when answering the question. A total of 402 amateur swimmers aged 6-17 years old who were local permanent residents (i.e., lived in the local area for half a year or more) were selected by random sampling to be subjects in this study. All subjects participated voluntarily and signed the informed consent form, in which younger children signed the informed consent form by their guardians.

Sampling and analysis
Sampling and questionnaires were conducted simultaneously. The water sample collection includes pool water and shower water. The air sample collection includes the air in the rest area around the pool and the air in the locker rooms. The sampling locations, which were disinfected by sodium hypochlorite, were the same as those where the questionnaire survey was administered.
Sampling of THMs in the pool water and air was performed eight times, independently, twice a week from July to August 2019. All of the sampling was conducted in the evening when the number of swimmers was usually the greatest for the day (Xiaolu et al. ).

Statistical analysis
The data obtained from the questionnaire were entered into an Epidata 3.1 (The Epidata Association, Odense, Denmark) database using the double entry method and logic checks. Data was cleaned and organized using Excel (Version

Exposure assessment
Based on the exposure factors and THM data collected in this study, an exposure assessment was conducted to evaluate the potential THM uptake via oral ingestion, inhalation, and dermal absorption. The equations for calculating the chronic daily intakes are shown below.

Oral ingestion pathway
According to the USEPA recommended model (USEPA ), the equation for estimating the exposure via direct oral ingestion from swimming pool water is as follows: where:

Dermal contact pathway
The exposure of swimmers to THMs through dermal contact absorption can be divided into two parts. One is the dermal contact absorption of THMs while swimming.
The other is the dermal contact absorption of THMs while taking a shower. Hence, the equation for estimating the exposure via dermal contact absorption is as follows: where:

Inhalation pathway
The exposure of swimmers to THMs through inhalation can be divided into two parts. One is inhalation of THMs while swimming, warming up and resting. The other is inhalation of THMs while taking a shower and changing clothes.
Hence, the equations for estimating the exposure by inhalation of THMs are as follows: where: where:

Noncarcinogenic risk assessment
The noncarcinogenic chronic toxicity of chemical pollutants in the swimming pool is generally measured relative to the reference dose (RfD). Exposure levels above the RfD may be dangerous. Exposure levels equal to or lower than the RfD are unlikely to be dangerous. In general, the noncancer risk is calculated as: HI der ¼ CDI der =RfD (12)  Table 2. Table 3 shows the concentration of THMs in swimming pool water and shower water. The source of the shower water at

Risk assessment of THMs
Lifetime cancer risk of THMs  They were all greater than 10 À6 , which is an unacceptable risk according to the USEPA Guidelines. This result indicated that children aged 6-17 years have a certain cancer risk when exposed to TCM in indoor swimming pools.

China. A comparison with the relevant data from other
swimming pool air samples is carried out and shown in Table 7. The concentration of THMs in swimming pool air in this study is close to that of samples taken in Tianjin     have a certain cancer risk when exposed to TCM through inhalation at indoor swimming pools. Unequivocally, the

CONCLUSIONS
In this study, we found that the cancer risk in children swimmers increased with age, caused by an increasing in exposure factors such as body weight, exposure time and skin surface area. Therefore, accurate determination of exposure factors are important for health risk assessment.
The research regarding exposure factors in swimming should be expanded upon to supplement the basic data in the exposure factors handbook.
This investigation further showed that the major exposure pathway for child swimmers was inhalation during swimming. It contributes to more than 97% of the total health risk. Hence, the risk of inhalation exposure while changing clothes, showering, doing preparatory activities and resting should also be included to make the risk assessment more comprehensive and complete. Conversely, the contribution through dermal absorption while showering is negligible.
Among the four species of THMs, TCM was the most abundant THM, comprising more than 96, 97 and 93% of THMs in swimming pool water, swimming pool air and locker room air, respectively, and leads to the greatest risk.
The risks from exposure to TCM through inhalation in children aged 6-17 years were all greater than 10 À6 , which is an unacceptable risk according to the USEPA Guidelines. This study showed that a decrease in the concentration of TCM to below 1 μg/m 3 in the swimming pool air can effectively reduce the risk of exposure to TCM through inhalation to within an acceptable level. Better management practices for the air at indoor swimming pools are necessary to reduce the risk to bathers' health. Therefore, the administrative departments and disease control authorities should pay attention to the hygiene and safety of swimming facilities and at the same time, establish standards for THMs in the air through further research.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.