Final answer:
For a positive fit check, breathe out while blocking the exhalation valve; for a negative fit check, breathe in while blocking the inhalation valve. Proper fit will prevent air leakage in both checks.
Step-by-step explanation:
For a positive fit check of a respirator, breathe out, while blocking the exhalation valve. This causes a positive pressure within the mask, and if the respirator fits properly, there should not be any leakage of air. Conversely, for a negative fit check of a respirator, breathe in, while blocking the inhalation valve. This creates a negative pressure inside the respirator, and again, there should not be any air entering the mask if the seal is correct.
Negative pressure is lower than atmospheric pressure, whereas positive pressure is greater than atmospheric pressure. A well-fitting respirator will maintain the proper seal and pressure during both inhalation and exhalation, helping to ensure that the wearer is protected from inhaling harmful substances.