Most air purifiers have an indicator light which comes on when it is time to change the air purifiers filter. These indicator lights are often set to a time period rotation so the filter will be reported as requiring changing regardless of how much dirty air has actually pass through it. Often as an extra qualification and to help you decide weather or not a air purifier filters need changing the manufacturers include a color-coded chart to help you get an idea as to how dirty the filter is.

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