School Vapor Intrusion Before a Heating-Season Decision
A 1960s elementary school in a Midwestern state sits 90 m from a former metal-degreasing shop that used trichloroethylene (TCE) until 2004. In winter 2024, sub-slab soil gas under the gym is 1,800 µg/m³ TCE; indoor air in the gym is 42 µg/m³ and in a first-grade classroom is 9 µg/m³. Outdoor background is 0.4 µg/m³. The school has 420 students, 38 staff, and a basement mechanical room with a sump that runs continuously. The district asks the health department whether to close the school immediately. The state indoor-air screening level for TCE is 2 µg/m³ (chronic, residential). No indoor air samples were collected during the previous heating season; the HVAC system runs 6 a.m.–4 p.m. and is off at night. Staff report solvent-like odors in the gym on cold mornings, and two teachers have chronic headaches, but no medical records are provided.
1. Calculate the indoor-air hazard quotient for the gym and the classroom using the state screening level, and explain what the calculation does and does not tell you about immediate health risk.
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Calculation
- Gym: HQ = 42 µg/m³ ÷ 2 µg/m³ = 21
- Classroom: HQ = 9 µg/m³ ÷ 2 µg/m³ = 4.5
What it means: Both values exceed 1, so TCE concentrations are above the chronic noncancer screening level. The screening level is derived with uncertainty factors and is intended to identify exposures that deserve further evaluation, not to predict an individual child's outcome. An HQ of 21 is a large margin above the level of concern and should trigger expedited investigation and interim exposure reduction.
What it does not mean: The HQ is not an acute toxicity threshold, a dose-response estimate for headaches, or proof that the teachers' symptoms are caused by TCE. It also does not prove vapor intrusion, because the samples are not paired with outdoor air, sub-slab gas, sump, or HVAC data. The calculation supports precautionary action, not a diagnosis or a closure decision by itself. A defensible decision would combine the HQ, the continuous sump pathway, winter stack effect, occupied sensitive population, and the need to confirm source and exposure before choosing relocation versus source control.
2. Design a 2-week sampling and building-diagnostics plan that can distinguish vapor intrusion from indoor sources before the district decides on relocation. Specify sample types, locations, timing, and quality-control steps.
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Sampling design
- Collect concurrent 24-hour indoor air samples in the gym, first-grade classroom, mechanical room, and a basement office, plus one outdoor upwind sample, using evacuated canisters and TO-15 analysis.
- Collect sub-slab soil gas beneath the gym and classroom, plus sump-pit headspace and a sealed-sump test; sample near the foundation perimeter to map the source pathway.
- Run the HVAC in normal occupied mode and then in off mode for paired samples; winter conditions are essential because stack effect and soil gas migration are greatest when the building is warm relative to soil.
- Record building pressure, temperature, wind, and sump operation during each run; inspect for indoor solvent storage, cleaning products, and recent flooring or furniture work that could explain TCE.
Quality control
- Include one field duplicate per 10 samples, one field blank per batch, and leak-check all canisters. Use certified labs and consistent reporting limits below 1 µg/m³.
- Compare indoor-to-outdoor and indoor-to-sub-slab ratios; a strong sub-slab-to-indoor gradient with a sump pathway supports vapor intrusion, while indoor sources produce elevated indoor air without a corresponding soil-gas pattern.
Decision use: Relocate sensitive occupants if confirmed indoor concentrations remain well above the screening level and source control cannot be completed quickly; otherwise seal the sump, improve depressurization, and resample.
3. Write the key elements of a risk communication message to parents that acknowledges uncertainty without minimizing the findings.
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Plain-language framing
- State what is known: TCE was detected in the gym above the state screening level, and the school is next to a former degreasing shop. State what is not known: whether the TCE is entering from soil gas or from an indoor source, and how concentrations vary over time.
- Explain the screening level in one sentence: it is a conservative long-term health-protection benchmark, not a threshold that predicts illness in a specific child.
- Describe actions already taken: additional winter sampling, sump inspection, HVAC review, and interim steps such as sealing the sump or restricting gym use if confirmatory results are high.
- Give a clear timeline and a named contact, and commit to sharing results even if they are inconclusive.
- Acknowledge symptoms without blaming them on TCE or dismissing them: advise parents to consult their pediatrician if concerned, and explain that headaches have many causes.
- Avoid 'the school is safe' and avoid 'there is no risk.' Use 'we are acting to reduce exposure while we confirm the source.' Offer translation and an in-person meeting.