The user of an air-purifying respirator must understand its limitations and recognize situations that require more extensive protection. Present contaminants do not have warning properties, e.g., odor, irritant properties or taste. Many contaminants have warning properties at high concentrations. By then it is too late. Odor, taste and irritation are easily determined.
Many compounds have no warning properties.
However manufacturers can inform you of specific warning properties. Detection of contaminants that do not have warning properties can be difficult and therefore, the contaminant can leak through or around a respirator and you won't know it. Manufacturers of specific chemicals can suggest respirators to be used for specific compounds. There is an unknown contaminant. This is the most frequently encountered danger in the work place.