HEPA Filter Efficiency Explained

Unfortunately it is poorly understood even by the very marketers trying to convince you to purchase their products. Certain misunderstandings about it are even exploited by competitors in order to confuse you and dissuade you from HEPA air purifiers.

Should the oft-touted HEPA filter efficiency rating really impress you? Should it have any bearing at all on your choice of air purifier?

My own personal decision is that HEPA filtration is the only sensible choice for home air purifiers. Please allow me to share many of the facts that led me to that decision.

A Simple HEPA Definition

HEPA is an acronym for "high efficiency particulate absorbing" or "high efficiency particulate arrestance" or, as officially defined by the Department of Energy (DOE) "high efficiency particulate air".

The differences are due to the fact that there is no single standard or certifying organization. This isn't necessarily a bad thing, but it can be confusing.

The phrase “True HEPA” is a loosely used marketing term. Various industries and institutions have different specifications to define HEPA. For example the European Standard EN1822-1 defines a HEPA filter as ranging from 85% to 99.995% efficient against a 0.3 micron challenge. The standard adopted by many industries is MIL-STD-282 Method 102.9.1 which requires the filter to capture 99.97% of 0.3 micron particles.

A HEPA air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria and any airborne particles with a size of 0.3 micron.

This doesn't mean it is the best a HEPA filter can do. It actually represents the worst it can do.

The specification of 0.3 micron is what is called the most penetrating particle size (MPPS). Particles that are smaller or larger are trapped with even higher efficiency. I'll show you how later.

Using the worst-case particle size results in the worst-case HEPA filter efficiency rating of 99.97%.

Why Is HEPA Filtration Efficiency So Highly Praised In Air Purifier Advertisements?

So let me give you an idea of where HEPA filtration, and only HEPA filtration, is used.

HEPA air filters have been traditionally used in hospital operating and isolation rooms, pharmaceutical and computer chip manufacturing.

HEPA air cleaners, vacuum cleaners and air filters are used in a wide variety of critical filtration applications in the nuclear, electronic, aerospace, pharmaceutical and medical fields.

HEPA air cleaners, vacuum cleaners and air filters are required by law to be used in all equipment for asbestos, lead, toxic chemical and mold abatement. These HEPA filtered products must meet the strict Military Standard 282 HEPA filtration efficiency test.

Some of the industries using HEPA filtration include:

• Microelectronics (eg. semiconductor cleanrooms)
• Pharmaceutical
• Bio and gene technology
• Chemical industry
• Nuclear air ventilation
• Waste incinerators
• Hospital operating rooms
• Emergency burn centers
• Cosmetics
• Medical industry
• Food industry
• Optical industry
• Automotive industry
• Surface engineering
• Precision engineering
• Nanomaterials
• Space industry
• Military equipment
• Power and energy plants
• Controlled and ultraclean environments for critical technologies
• Movie theatre industry
• Portable residential air cleaners

How Is HEPA Filtration Efficiency Achieved?

The filter media is pleated to provide a larger surface area. Close pleating, however, can cause particles to bridge the pleat bottom, reducing the surface area.

Better quality HEPA filters use separators between the folds to maintain even spacing and airflow across the filter and to prevent the media from collapsing.

HEPA filter efficiency is affected by fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 micron. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect.

HEPA filters are designed to target much smaller pollutants than the actual gap between fibers. HEPA filter efficiency depends on the following four mechanisms:

Direct interception works on particles in the mid-range size. These mid-sized particles follow the flow stream as it bends through the fiber spaces. Particles are captured when they touch a fiber. This effect is dominant from about 0.1 micron up to about 1 micron.

Inertial Impaction works on large, heavy particles. Inertial impaction occurs when larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly. This effect is dominant from around the 0.5 micron region up to around 5 micron.

Diffusion works on the smallest particles. Diffusion is perhaps the most mysterious of the filtering effects since it tends to defy common sense. Very fine particles in the air stream will collide with gas molecules and create a random path through the media. The smaller the particle the longer the particle will zigzag around. This random motion increases the probability of the particle contacting a fiber. This effect is dominant for all particles smaller than 0.1 micron.

Sieving stops large particles that are just too big to fit through the open areas of the filter. This includes all particles above 5 micron in size. As you go smaller in particle size, say between 1 micron to 5 micron, occasionally some of these particles get through, but the efficiency for removal is still well into the 99.9999+% range. This is due primarily to sieve effect and the beginning of inertial impaction effect.

Is HEPA Filter Efficiency Great Enough To Remove Bacteria and Viruses?

Why is HEPA Filter Efficiency Important In Home Air Purifiers?

However, none of these provide you HEPA filter efficiency. HEPA filter air purifiers are highly recommended for all allergy and asthma sufferers.

The American Lung Association recommends that source control be a primary means of reducing pollutants. However, studies show that HEPA air cleaners are effective in removing certain indoor air pollutants.

The reality in most homes is that total elimination of a pollutant source is not always possible. Individuals with severe allergy and asthma symptoms not alleviated by source control and ventilation may want to try a HEPA air cleaner to help reduce exposure. Although there is no proven health benefit, some individuals report that air cleaners are useful in improving their health.

HEPA filters are easily the most effective filters available and their use can improve allergic symptoms dramatically.

HEPA filters are more effective than any other type of air filter at capturing dust, pollen, ragweed, dust mites, mold spores and other allergens.

HEPA filters require no cleaning or maintenance to maintain efficiency, and studies have proven that HEPA filter efficiency actually increases with use over their 3 to 5 year life.