Which of the following values is the 15-minute time-weighted average exposure limit that should never be exceeded?

The 15-minute time-weighted average exposure limit that should never be exceeded is referred to as the TLV-STEL, which stands for Threshold Limit Value - Short Term Exposure Limit. This value is specifically designed to protect workers from exposures that could result in harmful effects from short-term exposure to hazardous substances.

The TLV-STEL is established to prevent potential toxic effects that could arise from high concentrations of a substance over a brief period, typically not to exceed 15 minutes. Exceeding this limit could increase the risk for adverse health effects, even if the long-term exposure levels (as indicated by TLV-TWA) are within acceptable ranges. Thus, it serves as an important guideline in occupational health to maintain worker safety during short bursts of exposure to hazardous materials.

In comparison, TLV-TWA refers to the average exposure over a standard 8-hour workday and is not specifically designed to address short-term spikes in exposure. PEL, or Permissible Exposure Limit, is often set by regulatory agencies and may also include both time-weighted averages and short-term limits, but the specific aspect of a 15-minute average is addressed by TLV-STEL. LD50 refers to the lethal dose of a substance that kills 50%