Most CPAP and BiPAP machines have two filter types. The coarse, often dark filter is the first line of defence and blocks fluff, hairs and other large particles. The often thinner, lighter coloured fine filters tackle smaller particles including dust, pollen and so on.
Fine filters should be replaced approximately every 2-4 weeks, depending on the air quality where you live. Coarse filters may be washable and should be rinsed or replaced at least every month.
I got this with the DeVilbis Auto PAP device in Jan 2010 and the two fit and work very well together. I am a first-time user of any kind of device.
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