The concentration that should never be exceeded is:

• A. TLV
• B. PEL
• C. TLV TWA
• D. STEL

Answer: (A)

In occupational exposure, several limits guide how much of a hazardous substance a worker can encounter. The TLV, or threshold limit value, is the guideline for safe exposure levels. It represents concentrations that workers should not exceed under normal conditions because exceeding them is associated with increased health risk. This makes TLV the best answer when asked for a concentration that should never be exceeded in daily work.

OSHA’s PEL is the legal limit, which can be different from the recommended safety margin; TLVs are used as practical guidance to prevent harm, not just to comply with law. The TLV-TWA is a specific form of TLV that averages exposure over an 8-hour shift, while the TLV-STEL is a short-term limit (usually a 15-minute exposure) within a workday. Those time-specific limits are important, but the general statement about not exceeding a safe concentration aligns with TLV as the overall guideline.