What respiratory protection type is not recommended unless conditions are well-known and maintained?

The choice of air purifying respirator as the answer highlights the importance of understanding the environment in which respiratory protection is used. Air purifying respirators rely on filters or cartridges to remove contaminants from the inhaled air. Their effectiveness is tied directly to the specific contaminants present in the atmosphere and their concentrations.

If the conditions within a confined space are not well-known or maintained, using an air purifying respirator can pose significant risks. For example, if the contaminant levels exceed the filter's capability or if an oxygen-deficient environment is present, the respirator will not provide adequate protection, potentially leading to serious health risks or fatality.

In contrast, respiratory protection options like self-contained breathing apparatus (SCBA), dual purpose SCBA, and supplied air respirators offer a higher level of safety in unknown or fluctuating conditions. These devices provide breathable air from a compressed source or an external supply, ensuring a safe breathing environment regardless of the contaminant levels.

Thus, air purifying respirators are not recommended in situations where conditions are uncertain, as their effectiveness relies on the knowledge and control of the environmental conditions.