Which statement correctly differentiates time-weighted average (TWA) and short-term exposure limit (STEL)?

• A. TWA is the 24-hour average; STEL is a 2-hour average.
• B. TWA is the 8-hour average; STEL is a 15-minute maximum exposure not to be exceeded more than four times per day.
• C. TWA is instantaneous exposure; STEL is monthly.
• D. TWA and STEL are the same.

Answer: (B)

Understanding how a time-weighted average and a short-term exposure limit differ helps you see why this distinction matters for safety. A time-weighted average reflects the average airborne concentration over a standard exposure period, typically an 8-hour workday, and it relates to protecting workers from long-term or chronic effects. A short-term exposure limit, on the other hand, addresses brief spikes in concentration and is defined over a short interval—usually 15 minutes—with an allowance not to be exceeded more than a few times per day (commonly four), allowing recovery time between spikes.

That’s why the statement describing the 8-hour average for TWA and a 15-minute maximum for STEL not to be exceeded more than four times per day best captures the difference. Other choices misstate the time frames (such as 24-hour or 2-hour for TWA or STEL) or mix up instantaneous or monthly references, which don’t reflect how TWA and STEL are used in practice.