PHE 540 Module 2 Exposure Pathways Short Paper Example

Stack, Wind, Window, Lung: Tracing the Exposure Pathway for Ethylene Oxide Near a Sterilizer

[Student Name]

Southern New Hampshire University

PHE 540: Principles of Environmental Health

Module Two Short Paper

[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.

Stack, Wind, Window, Lung: Tracing the Exposure Pathway for Ethylene Oxide Near a Sterilizer

An exposure pathway has five parts: a source, a medium that carries the hazard, a point where people meet it, a route into the body and a population that is exposed. If any part is missing, there is no exposure. This paper traces each part for ethylene oxide released by a contract sterilizer in a composite Tennessee Valley city, where the plant's stacks sit about 400 meters from apartments and an elementary school.

The Source

The plant sterilizes surgical kits, catheters and other devices that would be damaged by heat. Pallets are sealed in chambers, exposed to ethylene oxide gas, then moved to aeration rooms where the gas slowly leaves the products. Releases happen in two ways. Point sources are the stacks, which vent gas after it passes through control equipment. Fugitive sources are leaks from chamber doors, aeration rooms and the warehouse, where freshly sterilized products continue to give off gas. Air measurements near a Michigan sterilizer showed that total emissions can be substantial even when reported stack releases seem small (Olaguer et al., 2020), which suggests fugitive releases deserve attention.

The Medium: Air and Its Movement

Ethylene oxide is a gas at ordinary temperatures, so air is the only important medium. It does not settle onto soil or build up in food or water in meaningful amounts, and it breaks down slowly in the atmosphere, over weeks to months. Its concentration near the plant therefore depends mostly on how much is released and how the air moves. On windy, sunny afternoons, mixing spreads the gas quickly. On calm, clear nights, a layer of cool air near the ground can trap it, so concentrations nearby may be highest when people are asleep at home. Fugitive releases at ground level are especially affected, because they never rise above the trapped layer.

Background

Not all ethylene oxide in the city's air comes from the plant. Monitoring in Michigan found a background level of about 0.25 micrograms per cubic meter in urban air away from any sterilizer (Olaguer et al., 2020), from vehicles, other industrial sources and natural processes. Any assessment must separate the plant's contribution from this background, because residents cannot be protected from background by controlling the plant.

The Exposure Point and Route

The exposure points are wherever people breathe air carrying the gas: the apartment complex, the school playground and classrooms, nearby houses and the plant's own loading docks. Because ethylene oxide passes easily through building walls and windows, indoor air near the plant will carry much of the outdoor concentration, so staying inside offers little protection. The route is inhalation; skin contact and ingestion are negligible for residents.

Other routes were considered and set aside. Ethylene oxide does not linger on playground equipment or garden vegetables, and residents do not drink water near the plant that could carry it. Workers handling freshly sterilized products may have skin contact, but that falls under workplace rules.

The Exposed Population

Exposure depends on how much time people spend in the plume. Table 1 compares groups.

Table 1. Exposure by group

Note. Distances are from the main stack.

From the Lungs to DNA

Once inhaled, ethylene oxide is absorbed quickly into the blood and carried throughout the body. It reacts directly with DNA, forming adducts that can cause mutations. The EPA's assessment concluded that this mutagenic mode of action is central to its ability to cause cancer, with the strongest human evidence for lymphoid and breast cancers (Jinot et al., 2018). Because the gas acts directly on DNA, there is no assumed safe threshold for cancer, and exposure early in life is treated as carrying greater risk.

From Exposure to Dose

For cancer risk, what matters is long-term average exposure. The EPA expresses ethylene oxide's cancer potency as an inhalation unit risk, the added lifetime cancer risk per microgram per cubic meter breathed continuously over a lifetime, with a higher value applied when exposure begins in childhood (U.S. Environmental Protection Agency, 2016). A resident's exposure is therefore estimated as the average concentration at home, school and work, weighted by the hours spent in each, then averaged over the years they live nearby. That estimate, multiplied by the unit risk, gives the risk the final assessment will characterize.

Sensitive Life Stages

Because ethylene oxide damages DNA directly, the same air concentration is assumed to carry more cancer risk for a young child than for an adult. Children breathe more air for their body size, their cells divide rapidly and they have more years ahead in which a cancer could develop. The apartment complex houses many young families, and the school serves children aged five to eleven. These groups therefore deserve separate attention in the exposure estimate rather than being averaged into the adult population.

Measurements That Would Fill the Gap

Three kinds of data would complete the pathway. Twenty-four-hour air samples at the apartments, the school, a few homes and an upwind location would measure concentrations and separate the plant's share from background. Repeating samples across seasons and on calm and windy days would show how concentrations vary. And dispersion modeling, using the plant's reported releases and local weather, would extend the measurements across the neighborhood. Together they would give the risk assessment a defensible exposure estimate for each group.

Conclusion

Every link in the pathway is present: a source with point and fugitive releases, air that carries the gas, homes and a school within reach, inhalation and people who spend long hours nearby. The missing piece is measurement, which the risk assessment will need to supply.

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

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.