What is the TWA limit for diesel particulate matter (DPM)?

• A. 0.01 mg/m3
• B. 0.5 mg/m3
• C. 0.1 mg/m3
• D. 1 mg/m3

Answer: (C)

The main idea here is what a time-weighted average (TWA) exposure limit means for diesel particulate matter (DPM). A TWA is the average amount of contaminant a worker can be exposed to over a standard period, usually an 8-hour shift. For DPM in underground coal mine settings, the limit is set to protect long-term health because DPM is a carcinogen and can affect respiratory health with cumulative exposure. The value used is 0.1 mg/m3, measured as a mass of DPM in the air, averaged over the shift. This provides a practical, enforceable target to keep exposure at a level associated with reduced health risk.

Think of it this way: if a worker experiences higher concentrations at times during the shift, those spikes are weighed against the lower concentrations later, so the overall average should not exceed 0.1 mg/m3. This encourages control measures—such as better ventilation, upgrading to cleaner equipment, engine maintenance, or switching to electric options—to keep the air clean over the full shift.

Other numbers would either be impractically restrictive or insufficient to limit health risk; for example, a much lower value would be very hard to achieve consistently in a mine, while higher values would permit greater cumulative exposure.