How are nonpowered APR particulate filters graded for oil degradation resistance?

Nonpowered APR (Air-Purifying Respirator) particulate filters are graded for oil degradation resistance using the levels N, R, and P. This grading system is critical for determining the effectiveness of the filters in various environments, particularly those that may expose the filters to oil aerosols.

Filters designated as "N" are not resistant to oil, making them suitable for environments free from oil-based particulates. On the other hand, those classified as "R" can provide some level of oil resistance, meaning they can be used in environments where oil aerosols may be present for a limited time. Finally, "P" filters are considered oil-proof and can be used in settings with high levels of oil exposure without significant degradation of their filtering capabilities. This classification is significant for ensuring that users select the proper filter for their specific work conditions, promoting both safety and effectiveness in protecting against airborne contaminants.

In contrast, the other grading systems described in the other options do not align with the established nomenclature recognized by regulatory bodies for such filters, thus making them incorrect choices in this context.