Ceiling limit

Ceilings are used for substances such as strong irritants or fast-acting toxic gases, where even a brief peak can cause harm that an average would not catch. They are marked with a C on exposure limit tables. OSHA, NIOSH and ACGIH each publish ceilings for some chemicals, and the type of limit for the same chemical may differ between them.

In practice, a ceiling cannot be checked by a full-shift sample alone. Where an instantaneous reading is not practical, the limit may be assessed using a short sampling period, such as a 15-minute sample, unless the standard specifies otherwise. The SDS or the underlying rule will indicate which applies. Direct-reading instruments and alarms are often used to watch for peaks.

A ceiling differs from the Short-term exposure limit (STEL), which is a 15-minute average, and from the Time-weighted average (TWA), which averages over a shift. If a substance has both a TWA and a ceiling, workers must meet each of them. The ceiling applies even when the average is low.

Because a ceiling is breached by a single peak, controls need to focus on tasks with sudden release: opening vessels, connecting hoses, sampling lines, charging materials. Local exhaust, closed transfer systems and alarms on fixed detectors are common measures. Respirator selection should assume the peak concentration, not the average.