I just bought, two used Austin Air HM-400 machines, and tried them out with these new filters from Amazon. These filters are high quality, just like the Austin Air units themselves. With coronavirus, allergies, pets, cigar smoke (my bad habit that I confine to one room of the house with an exhaust fan), and an asthmatic boyfriend, I need all the help I can get with improving air quality. Austin Air delivers. I’ve owned four other air purifiers, none have left the air smelling this clean. The machines clean the air very quickly on high in a normal size room. These machines will save me money over time not having to replace filters as often. I found out one of the machines I bought is 24 years old. It works just like new. The filters are worth the price.