PHE 540 Module 5 Milestone Two Example

Three Steps Toward a Number: Hazard, Dose-Response and Exposure for Ethylene Oxide Near a Sterilizer

[Student Name]

Southern New Hampshire University

PHE 540: Principles of Environmental Health

Final Project Milestone Two

[Instructor Name]

[Date]

The organization, setting and figures below are a composite written as a model document. No real employer, client, colleague or patient is described.

Three Steps Toward a Number: Hazard, Dose-Response and Exposure for Ethylene Oxide Near a Sterilizer

Milestone One planned an assessment of the added lifetime cancer risk from ethylene oxide released by the sterilization plant. This milestone completes the first three steps of the four-step framework: it asks whether the chemical can cause harm, how risk changes with dose and how much residents actually breathe.

Hazard Identification

Three lines of evidence show that ethylene oxide causes cancer. In people, the large cohort of sterilization and production workers followed by NIOSH showed excess lymphoid cancers among the most exposed (Steenland et al., 1991), and a study of 7,576 women in the same industry reported more breast cancer among women with greater cumulative exposure (Steenland et al., 2003). In animals, inhalation caused several tumor types in rats and mice. Mechanistically, ethylene oxide reacts directly with DNA and causes mutations, which explains how it can start cancers and why no safe threshold is assumed.

Weighing this evidence, the EPA judged in 2016 that inhaled ethylene oxide causes cancer in people, pointing above all to cancers of the lymphoid system and the breast (Jinot et al., 2018), and the International Agency for Research on Cancer also classifies it as carcinogenic to humans. Noncancer effects, such as nervous system and respiratory effects, occur at much higher concentrations than those expected near the plant and are not quantified here.

Dose-Response Assessment

For carcinogens assumed to have no threshold, dose-response is expressed as a slope: added risk per unit of exposure. The EPA derived an inhalation unit risk of 3 × 10⁻³ per microgram per cubic meter for exposure in adulthood and, applying adjustments for greater sensitivity in early life because ethylene oxide is mutagenic, 5 × 10⁻³ per microgram per cubic meter for exposure from birth (U.S. Environmental Protection Agency, 2016). These values come from the worker cohort's lymphoid and breast cancer data, extrapolated to low concentrations with a linear model.

The numbers mean that breathing 1 microgram per cubic meter continuously over a lifetime from birth would add an estimated 5 cancers per 1,000 people. A one-in-a-million added risk, a common benchmark, corresponds to about 0.0002 micrograms per cubic meter, far below the urban background of about 0.25 measured in Michigan (Olaguer et al., 2020). This is a major uncertainty: the slope implies that background alone carries a lifetime risk near one in a thousand, a conclusion some scientists dispute.

Why a Linear Model

The choice of a linear, no-threshold model follows from the mechanism. Because ethylene oxide damages DNA directly, even a few molecules could in principle start a mutation, so risk is assumed to fall in proportion to exposure all the way to zero rather than disappearing below some level. Critics argue that the body repairs low levels of DNA damage and that natural ethylene oxide formed inside the body exceeds what residents inhale from the plant. The EPA considered these arguments and kept the linear approach; this assessment follows it while reporting the debate as part of the uncertainty.

Early-Life Sensitivity

The higher unit risk for exposure from birth reflects standard adjustments for mutagenic carcinogens, which weight exposure before age two by a factor of ten and from ages two to sixteen by a factor of three. These adjustments matter for the apartment complex, where many residents are young children, and they will be applied to the child scenario in the final assessment.

Exposure Assessment: Methods

Twenty-four-hour air samples were collected every six days at five sites over three seasons: an upwind background site, the apartment complex 400 meters downwind, the elementary school 450 meters away and residential streets at 1,000 and 1,500 meters. Samples were analyzed by a certified laboratory. The plant's increment at each site was estimated by subtracting the background mean.

Exposure Assessment: Results

Table 1 summarizes the results.

Table 1. Mean ethylene oxide concentrations, micrograms per cubic meter

Note. Composite results; each mean is based on about 45 samples.

Concentrations fall steeply with distance. Samples on calm nights at the apartments ran two to three times the annual mean, consistent with trapping near the ground.

Exposure by Group

Long-term exposure depends on where each group spends its time. Table 2 shows time-weighted plant increments and assumed durations.

Table 2. Time-weighted plant increment by group

Note. Away-from-home hours are assigned the background-only increment of zero.

Comparison With Other Communities

The plant increment at the apartments, about half a microgram per cubic meter, is lower than the peak increment of nearly two micrograms estimated near the Michigan sterilizer (Olaguer et al., 2020) but well above the one-in-a-million level. The steep decline with distance resembles patterns reported around other point sources and suggests that the neighborhoods within about 500 meters carry most of the plant's added exposure. Those neighborhoods are also, as Module Four found, where renters, lower-income households and young families are concentrated.

Data Gaps

No indoor samples were taken, so indoor concentrations are assumed equal to outdoor ones. No samples were taken at the plant fence line. Both gaps should be filled before final decisions, and the risk characterization will note them.

Uncertainty in the Exposure Estimates

One year of sampling may not represent long-term emissions, which can change with production. Twenty-four-hour samples miss short peaks. Time patterns are assumptions, not diaries. And the background at the upwind site may itself include small amounts from the plant on days when wind shifted. The final assessment will present a range using the lowest and highest seasonal means.

Summary

Ethylene oxide is a known human carcinogen with a published unit risk, and residents near the plant breathe measurable increments above background, highest for children in the apartments. The final project will combine these exposures with the unit risks to characterize risk and recommend actions.

References

Jinot, J., Fritz, J. M., Vulimiri, S. V., & Keshava, N. (2018). Carcinogenicity of ethylene oxide: Key findings and scientific issues. Toxicology Mechanisms and Methods, 28(5), 386-396. https://doi.org/10.1080/15376516.2017.1414343

Olaguer, E. P., Robinson, A., Kilmer, S., Haywood, J., & Lehner, D. (2020). Ethylene oxide exposure attribution and emissions quantification based on ambient air measurements near a sterilization facility. International Journal of Environmental Research and Public Health, 17(1), Article 42. https://doi.org/10.3390/ijerph17010042

Steenland, K., Stayner, L., Greife, A., Halperin, W., Hayes, R., Hornung, R., & Nowlin, S. (1991). Mortality among workers exposed to ethylene oxide. New England Journal of Medicine, 324(20), 1402-1407. https://doi.org/10.1056/NEJM199105163242004

Steenland, K., Whelan, E., Deddens, J., Stayner, L., & Ward, E. (2003). Ethylene oxide and breast cancer incidence in a cohort study of 7576 women (United States). Cancer Causes & Control, 14(6), 531-539. https://doi.org/10.1023/A:1024891529592

U.S. Environmental Protection Agency. (2016). Evaluation of the inhalation carcinogenicity of ethylene oxide (CASRN 75-21-8) in support of summary information on the Integrated Risk Information System (IRIS) (EPA/635/R-16/350Fa). U.S. Environmental Protection Agency.