Chlorine Cylinder Leak Near a School
At 06:00, a water treatment plant discovers a leaking 1-ton chlorine cylinder at its chlorine room. The valve is damaged and the estimated release rate is 0.5 kg/min. Wind is from the west at 3 m/s, sky is clear, temperature is 10 C, and a low-level inversion is present. The plant is 400 m west of a residential neighborhood of 2,000 people. A school is 1.2 km directly downwind. The only paved evacuation route runs east-west and crosses the projected plume. The plant has SCBA but no chlorine repair kit on site; the local fire department is 12 minutes away. ERPG-2 is 3 ppm and ERPG-3 is 20 ppm.
1. Recommend protective actions for the 400-m neighborhood and the school during the first 30 minutes, and justify the choice between shelter-in-place and evacuation.
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The plume is moving east toward both receptors under stable, low-wind conditions. Stable conditions suppress vertical mixing, so chlorine, which is denser than air, will tend to travel near the ground and remain concentrated. At 0.5 kg/min, the release is not a massive catastrophic failure, but it is continuous and the valve is damaged, so the duration is unknown.
For the 400-m neighborhood, shelter-in-place is the safer initial action. There is little time to move 2,000 people, the evacuation route crosses the plume, and outdoor movement would increase exposure. Residents should close windows and doors, shut off HVAC, seal gaps, and move to an interior room on an upper floor if possible. For the school at 1.2 km, there may be enough time to shelter rather than evacuate; sheltering is still preferred until monitoring or modeling shows the plume is not reaching the school. Evacuation, if ordered, should move people north or south, not east through the plume.
Establish command upwind, isolate at least 150 m in all directions, notify dispatch and the LEPC, and send a reconnaissance team in Level B with SCBA. Do not commit responders to stop the leak without a chlorine kit and backup.
2. A responder argues that entry without SCBA is acceptable because chlorine odor is detectable well below the IDLH and the leak is small. Evaluate this argument and state the entry requirements.
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The argument is unsafe. Odor threshold is not a reliable exposure indicator: chlorine causes olfactory fatigue, and the ability to smell it varies widely. The IDLH is 10 ppm, and ERPG-3 is 20 ppm; those levels can be reached close to the leaking cylinder even when the release rate is only 0.5 kg/min. Chlorine is heavier than air and will accumulate in low areas, so a reading at breathing height may understate the hazard at floor level.
Entry to the chlorine room must use SCBA and chemical-protective clothing appropriate to the task, with two-out/two-in or equivalent rescue capability, continuous monitoring, and a backup team. The entry team should use a chlorine repair kit, not attempt to tighten a damaged valve with ordinary tools. If the kit is unavailable, the correct action is to isolate, ventilate only if it does not spread the plume, and wait for the fire department or a chlorine specialist. The responder should not enter based on odor or on the small estimated leak rate.