This article is the first in our new series about air purification technologies. It concerns the air purifiers with HEPA filters efficiency.
What is an air purifier with filters?
An air purifier with filters is a device intended to reduce or eliminate industrial and domestic pollution. It consists of several superimposed filters and covers a surface of area of about 5 to 150 m².
- The pre-filter job is to capture large particles (> 1 µm)
- The activated carbon filter tend to eliminate VOC and unpleasant odors
- The High-Efficiency Particulate Arrestance Filter removes (from the air that passes through) 99.97% of particles that have a size of 0.3 µm.
The HEPA filters and their theoretical efficiency
The « HEPA » appellation is often presented to consumers as a guarantee of excellence. This special designation is given only to product that have successfully undergone the DOP (Dispersed Oil Particulate) test. This test, developed in the 50’s, is used to test the filters ability to remove 99.97% of all particles of 0.3 microns mean diameter. In the air purifiers markets, there are a lot of products that do not necessarily meet the HEPA standard. Products that claim to be “HEPA-type”, “HEPA-like”, “HEPA-style” or “99% HEPA” may not have been tested in laboratories. Producers would play consumers for fools.
Their efficiency in practice
The Test Dop does not allow to determine the filters ability to trap particles smaller than 0.3 micron, which represent 90% of suspended particles. Concerning products that meet the HEPA standard, they are often inefficient on particles smaller than 0,3 µm, i.e on 90% of particulates. Then, filtration efficiency and purification are not synonym. The air speed through a filter must be fast if the purifier is to work well. So, the device becomes noisy.
A lot of factors limit the efficiency of HEPA filters :
- The type of filtering layer
- The temperature
- The air speed that passes through the filter : if it is slow, the room air is not totally purified ; if it is fast, the device becomes noisy.
- The wear : an HEPA filter loses 25% of efficiency in the space of six months, in relatively polluted environment
- The level of filter obturation by deposition of particles
According to a recent Canadian study, air purifiers with HEPA filter remove 68% of ultrafine particles and 63% of fine particles.
A very specific maintenance
HEPA filters require a maintenance routine in order to work efficiently.
- The filter should be cleaned every 3 – 4 weeks.
- The filter should be replaced at least once a year. Chinese bloggers recommend to replace it several times a month, during pollution peaks.
- Producers recommend to wear mask and gloves. Indeed, pollutants can disperse easily.
- Those who suffer from asthma are recommended not to handle filters.