The use of N-Series Respirators is typically limited only by hygiene, damage, and increased breathing resistance; however, when working in very dirty or dusty workplaces that may result in high filter loading (200 mg), service time should be limited to eight hours of continuous or intermittent use, unless an evaluation is done to prove that extended use will not significantly degrade efficiency levels. R-Series Respirators should only be used for one working shift (or for eight hours of continuous or intermittent use) when oil is present; however, when oil is not present, there is no specific service time. The use of P-Series Respirators is limited only by hygiene, damage, and increased breathing resistance. Welding Approved Respirators offer unique protective layers that are designed to be flame-resistant as demonstrated per ASTM D2859-96 (#70015, 80649 do not meet ASTM D2859-96).