How do you know whether a HEPA filter is really doing its job? When would you change the filter?

Q: How do you know whether a HEPA filter is really doing its job? When would you change the filter?
A: An air cleaner is a different product than for example, a heater or an air conditioner. With a heater or air conditioner, you feel heat or cool instantly. With an air cleaner users typically notice a decrease of allergic reactions or other symptoms over time - generally within 2-3 days of continuous use. Or they may notice they have less house dust (a mite allergen home) or they sleep better. Observing the filter also indicates the air cleaner is working, as the filter turns color as contaminants build up.
Most manufacturers have a device to signal when to change the filters. Better quality products use pressure switches that actually measure the point when filters become clogged, and need changing. This pressure switch is usually for the main filter. Less expensive products use a simple timer to approximate when the filter might be clogged.
Consumers can also feel where the air comes out. If it appears no air is coming out, or the air is "puffing" the filter probably is clogged, and needs to be changed. Interview with William Schuettenberg, Vice President, Healthway Products AllergyBuyersClub.com