This study established a Korean national representative exposure factor database for children’s product usage patterns using proportional quota sampling. The target population for a single survey was 10,000, and substantial respondents for the smallest population area and rarely used products were considered. Among the 10,000 respondents of 300 quotas in a single survey, the smallest quota was 7 for children aged 0 years in the county area of Gyeonggi. Conversely, the largest was 125 for boys 7–9 years old in the city area of Gyeonggi. The product with the lowest use rate (skateboard in the summer survey) had 570 respondents in a single survey. These sampling methods and sample sizes enabled the collection of representative exposure factors for children’s products.

Children’s age group division is an important component in the management of children’s products. The main consideration of this study was the key living space Korean children mainly stay or perform activities. For 0–2 years, their major living space was their own home. For 3–6 years, it was their own home and childcare facilities, such as daycare centers and kindergartens. For 7–12 years, it was their own homes and elementary schools. The age quotas of the preschool children were divided into two-year-old units except for babies aged 0–12 months. Elementary school children’s age quotas were divided into three-year-old units by grade level as lower- and higher-grade elementary school children might differ in lifestyle and product usage patterns. Finally, exposure factors were presented for the four age groups: infants aged 0–2, toddlers aged 3–6, lower-grade elementary children aged 7–9, and higher-grade elementary children aged 10–12.

In the Special Act on the Safety of Products for Children, Toy Safety Directive 2009/48/EC, and Safety Standard Mandating ASTM F963 for Toys of Korea, the EU, and the U.S., respectively, children’s products for children under 3 years (36 months) were more strictly regulated than products for 4–12 years. The age groups varied depending on specific considerations. The age group recommendations for monitoring and assessing childhood exposures considering behavioral physiological changes were in subgroups of <12 months, 1, 2, 3–5, 6–10, and 11–15 years [34]. In consideration of the consumer product skill characteristics, play behaviors, and interest the age groups of children were divided into subgroups of <12 months, 12–18 months, 19–23 months, 2 years, 3 years, 4–5 years, 6–8 years, and 9–12 years [35].

The national exposure factor handbooks in the US [21], China [36], and Korea [26] reflect these considerations and provide exposure factors for each parameter by children’s age group. In the Korean handbook, children’s body weight and total body surface area were classified according to three-month units before 12 months, and one-year-old unit. The body part surface areas of children aged 0–12 months, 1–2, 3–6, 7–9, 10–12, 13–15, and 16–18 years were also provided. The exposure factors of the children’s product usage patterns obtained in this study were matched with the physiological exposure factors in the Korean handbook and could be used for the management of children’s products in risk assessments.

The exposure factors from the nationwide survey were used for a more refined exposure assessment. In the case of products that can be compared directly, default values of the RIVM toys fact sheet [24] for ballpoint pen (1 event/day, 30 min/event) and marker pen (4 event/week, 30 min/event) was much higher than values observed in this study for ballpoint pen (overall 0.40 ± 0.24 event/day, 15.0 ± 15.7 min/event) and marker pen (overall 0.31 ± 0.19 event/day, 14.2 ± 13.8 min/event). The default values of the toy fact sheet derived from the worst-case assumption by a single value had a low quality factor from the RIVM’s quality grading of data reliability [22, 25] for quantitative exposure estimation. In contrast, the exposure factors obtained from the national representative survey in this study had a high quality factor, indicating reliability of the estimation.

The data of products that have undergone risk assessment studies were used for the comparison of exposure factors used for exposure estimation. Several studies on diapers and wet wipes related to skin contact, baby bottles related to oral intake, and teethers and pacifiers related to mouthing activity have been conducted for infants under 36 months. Product usage pattern information about sporting goods for children has rarely been reported. For stationery, usage pattern information of school children was reported for some arts and craft materials.

The frequency of disposable diaper use obtained from the survey [37] was comparable to the results of this study. The mean ranged from 0.3 in India to 5.9 in the U.S., which was slightly lower than the results of this study (overall 6.05 ± 2.87 event/day). The 95th percentile in Japan and the U.S. was 10, the same as this study. The use rate of wet wipes as a baby care product was 87%, and the use frequency was 3 times per day [38] for parents of children aged younger than 3 years in the U.S. In this study, Korean children used wet wipes more often, with a use rate of 96.5% and a mean use frequency of 6.63 ± 4.24 event/day in aged 0–2 years.

The daily bottle-feeding frequency reported to be in the range of 0–7.5 times and mean of 2.3 ± 1.8 times in children aged 12 months [39] was comparable to the use frequency of baby bottles (overall 3.75 ± 1.91 event/day) in Korean children aged 0–2 years of this study. In the United Kingdom, the daily pacifier use duration of children aged 24–71 months was 3.6 h/day [40]. In this study, the average daily pacifier use duration derived from the mean values of use frequency (overall 2.80 ± 1.95 event/day) and use duration (overall 17.58 ± 11.60 min/event) of Korean children aged 0–2 years was 0.82 h/day. For teethers, a mouthing time of 23–27 min/h for soft plastic articles [11] was used to exposure estimation, and one-time exposure (exposure frequency, EF = 1) assumption [41] was used for risk assessment.

The use rates of drawing materials for preschoolers obtained from a survey in Lithuania [42] were 80.1%, 47.9%, and 38.0% for colored pencils, watercolors, and chalks, respectively. In the U.S., 20% of children used markers for at least 30 min at a time [43], and the school children’s mean use durations for crayons, glues, and pencils were 5.9, 3.97, and 5.83 h/month, respectively [44]. Korean children participated less in drawing activities than U.S. children. In this study, children aged 7–12 years old used oil pastels, colored pencils, marker pen, and pencils at an average of 3.55, 3.74, 7.87, and 18.38 h/month, respectively. The monthly use durations of oil pastels and colored pencils were derived from the mean value of use frequency (event/day) and use duration (min/event). In this study on some stationery products, such as notebooks, pencils, ballpoint pens, and marker pens, directly investigated the daily use duration (min/day); hence the use frequency (event/day) of these products was difficult to specify.

The exposure factor results presented in this study can be used as input parameters for refined exposure assessment considering children’s sex, age, and season. Among sex, age, and seasonal differences, the most obvious difference was that among age groups. Sex differences were rarely observed in infants. The seasonal differences in use rate and time may be related to outdoor activities as the products with a 20% higher use rate in summer were beach balls, picnic mats, swimming goggles, lunch boxes, baby bottles, and bubble-making toys. In the case of beach balls and picnic mats, their use duration in summer was 9.1 and 20.5 min/event higher than that in winter. The remaining four products had similar use durations in summer and winter. In addition, the average differences in use frequency for these products were small, ranging only 0.02–0.20 event/day, despite the significant differences. These results show that the seasonal difference was mainly affected by the use or non-use of a specific product, and that the quantitative exposure factors as input parameters of exposure estimation were less related.

The exposure factor was investigated by considering the product characteristics and the exposure algorithm. The determination of the single event and use duration (min/event) of some stationery products, such as erasers, correction tape/fluid, glue, adhesives, and scissors were difficult. For wet wipes, the use amount (ea./event or g/ea.) was more important for quantitative exposure estimation than use duration. Food containers, such as food trays, lunch boxes, and water bottles, had difficulty defining contact event with the human body. The concentration characteristics by material, such as the migration level, would be more important for risk assessment than product usage pattern characteristics.

The exposure factor survey in this study was conducted twice, once in both summer and winter; however, the same population was not surveyed. As the period between the summer and winter surveys was eight months, some respondents had to change age groups due to the growth of the children. The derived seasonal difference might be caused by the difference in seasons and between respondents. Also, the quota sampling method had a potential for selection bias. It was not accessible to those who are not initial 40,000 panels. The sampling error cannot be assessed since the sample was not randomized. Nevertheless, the 300 quotas which considering the sex, age, and regional distribution children are sufficiently representing Korean children’s population. Deriving nationally representative results was still possible.

The survey questionnaire was developed for the children’s parents. Although elementary school children responded with their parents, parents’ responses might not have accurately described their children’s product usage patterns. This was a fundamental limitation of the survey method and not of the observation method. However, face-to-face interview surveys are more suitable than observations in investigating the usage patterns of children’s products for various categories due to resource and financial restrictions. There was no verification about children’s behavior with products at childcare facilities or school. Further surveys for children’s products will be able to select additional products based on the actual observation in children’s activity space and interview with the kindergarten or school teacher.