PHEL curves provide information about safe durations under varying workloads. Which option best describes this?

• A. Safe exposure durations
• B. Air humidity ranges
• C. Noise exposure limits
• D. Recovery heart rate targets

Answer: (A)

PHEL curves describe how long you can stay at a given workload before safety or performance risks become unacceptable. The key idea is that the allowed duration depends on how intense the workload is: at lighter effort you can endure longer, while at higher effort you must limit the time dramatically. This relationship helps plan work-rest cycles and ensure safe operation across different task intensities.

That’s why the best description is safe exposure durations—because the curves are about the maximum time permissible at various levels of workload. The other options don’t fit: environmental humidity ranges aren’t about how long a workload can be sustained; noise exposure limits focus on sound levels and time-weighted exposure, not general workload duration; recovery heart rate targets pertain to post-exercise monitoring rather than the duration you can safely remain under a certain workload.