When Should an Air Purifier Filter Be Replaced?

1. Measure performance

Use a PM2.5 meter to measure the PM2.5 concentrations at the air inlet and outlet and calculate purification efficiency. Use an anemometer to measure outlet air speed and calculate the current CADR as a percentage of its initial value. Replace the filter immediately if that figure falls below 50%. Outlet airflow can be estimated by comparing outlet air speed at the same fan setting.

For example, a new unit may have an outlet speed of 1.5 m/s and 100% purification efficiency on its first setting. After a period of use, the outlet speed on the same setting may be 1.2 m/s and purification efficiency 80%. The current CADR is then 1.2 × 80% / (1.5 × 100%) = 64% of its initial value. Measure outlet speed at the same point each time.

Purification efficiency = [1 - (PM2.5 concentration at the outlet / PM2.5 concentration at the inlet)] × 100%

2. Observe the filter color

A HEPA filter has two sides. The side where air enters is the front, and the side where air exits is the back.

HEPA filters are generally white, slightly yellow, green, or silver when an antibacterial material is added. Replace the filter when the back turns dark gray or black.

As the filter is used, particles become embedded in the filter fibers and the material gradually darkens. The position where particles accumulate varies with the filter grade. Higher-grade filters fill from the front first, while lower-grade filters fill from the back first. The back of a higher-grade filter is therefore less likely to turn black.

For high-grade filters of H11 and above, inspect the back. For lower-grade filters, such as coarse and medium-efficiency filters rated F9 or below, inspect the front.