Which statement best describes the relationship between STEL and TLV-TWA?

• A. STEL replaces TLV-TWA for long-term exposure.
• B. STEL is used only for skin exposure.
• C. STEL equals TLV-TWA.
• D. STEL supplements TLV-TWA when acute effects are observed from high short-term exposures.

Answer: (D)

Short-term exposure limits are about preventing immediate, acute health effects from brief spikes in concentration, while TLV-TWA sets a limit on what’s acceptable when averaged over a full work shift. The STEL is a separate, usually 15-minute limit that catches those quick peaks that could cause irritation or other acute responses. They work together to protect workers: you keep the long-term average under TLV-TWA and also ensure that no short-term interval exceeds the STEL. When there’s a risk of acute effects from high short-term exposures, the STEL provides additional protection beyond the TLV-TWA, rather than replacing it. That’s why the statement that STEL supplements TLV-TWA when acute effects are observed from high short-term exposures is the best description.