HEPA Filters FAQ
There are a number of misconceptions held and near truths told about vacuum cleaner filtration in the cleaning industry. This guide is meant to clarify the details about HEPA filtration and to inform about the way in which high efficiency filters must be used.
What Does HEPA Mean?
HEPA filter means High Efficiency Particulate Air filter. The filter for filtering out particles contained in an air stream, at high efficiencies.
What is High Efficiency and How Do You Test the Efficiency?
HEPA filters work at efficiencies above 99.9%.
A special test dust is used to check the efficiency. Normally this test is done at specific airflow values dependent on the filter surface area. There are different tests and different standards used for different industry requirements. HEPA filers used for personal protection are tested in a specific way, HEPA filters in high velocity, low volume applications (vacuum cleaners) are tested using a different standard and low velocity, high airflow filters utilize a different standard.
Part of the problem with standards is there is no standard standard! Different countries and different industries will utilize different standards.
The good news is that there is an international standard that is being developed. The participation countries are currently debating this standard, but it hopes to clearly define test methods for filtration efficiency for HIGH, MEDIUM and LIGHT hazardous dusts.
The bottom line is that when filtration efficiencies are claimed the company making the claim should be able to disclose complete details regarding the standard used.
One of the most important considerations in testing efficiency is filter surface area and airflow. For vacuum cleaners the filter should be tested at the maximum flow rate of the product, with the product operating via its own motor.
What is a Particle?
A particle can be anything from a smoke particle to a grain of sand to a rock.
It makes intuitive sense that it is harder to filter particles that are smaller in size. Keep in mind that not only does the size of the particle matter, but so does its velocity. A fast moving particle of a given size is harder to filter than the same particle at a lower speed.
When filtering particles in the sub-micron range (less than 1 micron) it is not only the size of the particle that matters but also its shape and mass. In particular, particles at 0.3 microns have a lot of penetrating power.
What is a Micron?
A micron is a metric unit of measure, which is one millionth of a metre. A metre is roughly 39.5 inches.
For comparison, the average diameter of a human hair is 100 microns and the smallest particle the human eye can discern is 10 microns.
It is important to know that the human upper respiratory tract is only capable of stopping particles that are 10 microns in size and larger. Particles smaller than 10 microns are considered 100% respirable meaning they escape the body’s defense mechanisms.
What is Test Dust?
A test dust is a dust containing particles of a known size and distribution.
One of the most common test materials is DOP (dispersed oil particulate) smoke. Another common one is sodium chloride test dust.
What Do the Tests Measure?
Normally, overall filter efficiency is measured against the full distribution of particles in the test dust. If 100 milligrams of dust goes in 0.1 milligrams comes out, the efficiency is 99.9%.
If required, it is also possible (by a particle counting and sizing method) to establish the efficiency at a particular particle size.
It is important to know that the filter test takes only a few minutes and the volume of test dust is very small. The test measures the efficiency during the test and does not predict how the filter will perform in actual use. Thus it does not test how long the filter will last before:
a) it clogs up and lets no air pass
b) it clogs up and particles pass through
c) it clogs up and air leaks past it
It is for this reason that careful design considerations are necessary to have a HEPA vacuum, which is an efficient product and not just a product with an efficient filter.