HEPA (high efficiency particulate air) filters are air filters that remove particles from the air by forcing air through screens with microscopic pores. This makes them highly effective at reducing the number of allergens and asthma triggers (e.g., dust, pollen, mold, dander, tobacco smoke) in an enclosed space. Originally designed for industrial, medical and military use, HEPA filters are now being used to clean the air in individual homes and offices.

There are two types of filters that bear the HEPA name:

• True HEPA filters
• HEPA-type filters

A True HEPA filter must be capable of removing 99.97 percent of the particles that are 0.3 microns or larger from the air. It must allow, on average, only 3 particles out of every 10,000 to pass through the filter. This means a HEPA filter is very effective at filtering out allergens and asthma triggers. HEPA-type filters use the same kind of technology as True HEPA filters. However, they are generally cheaper and do not filter out particles as effectively. The efficiency rating on a filter is a very good indication of its performance.

HEPA filters are generally expensive, and most types are sold in standalone versions that clean the air in a single room. Though they are available, whole-house cleaning systems are often not in the price range of the average consumer. HEPA vacuum cleaners, which are capable of removing allergens from the surfaces on which they are used, are also available.

HEPA filters were developed in the United States during World War II to stop the release of radioactive particles from the exhausts at nuclear reactor facilities. Since that time, HEPA filters have been further developed for use in industrial, medical and military clean rooms where small particles can interfere with research or technology development (for example, facilities that build microchips). Today, HEPA filters are becoming increasingly popular in homes in the form of portable residential air cleaners.

HEPA is not a brand name. It stands for High Efficiency Particulate Air filter. Many different types of HEPA filters are sold under many different brands. The filters are designed for use indoors, and most consumer models are designed to be used in just a single room.

HEPA filters work by forcefully pushing air through filters made of microscopic glass fibers. These filters are very similar in thickness and texture to blotter paper. Because the filters are so effective at blocking particles from passing through, they often cause a great deal of resistance to airflow. To deal with this phenomenon, HEPA filters often require a powerful fan or blower to force air through the filters.

Because of their airflow requirements, HEPA filters are generally not suitable for home air conditioning systems. Implementing a house-wide HEPA filtration system would be very expensive. Instead, HEPA filters are often sold in standalone units for individual rooms.

HEPA filter technology is also becoming popular on some specialized vacuum cleaners. This allows vacuums to remove dander and other allergens instead of stirring them up into the air (which often occurs with normal vacuum cleaners). Once airborne, these allergens can trigger allergies or asthma when inhaled. HEPA vacuum cleaners are generally more expensive than regular models, but generally less expensive than HEPA systems designed to filter the air in a given room.

There are two types of filters that bear the HEPA name:

• True HEPA filters
• HEPA-type filters

HEPA-type filters are generally cheaper than True HEPA filters, but do not filter particles as effectively. True HEPA filters are considered extremely effective at removing small particles from the air because they are able to stop 99.97 percent of those particles that are 0.3 microns in diameter or larger. A micron is one-millionth of a meter, meaning it would take about 500 microns to span the period at the end of this sentence. A particle 0.3 microns in diameter is far too small for the eye to see. With this type of efficiency, all types of airborne allergens and asthma triggers can be eliminated or greatly reduced with the use of a HEPA filter.

HEPA filters are effective at removing the following allergens and particles from the air:

• Pollens. Small, powdery grains of flowering plants that can easily become airborne. These are most often produced by trees, grass and weeds.
  
  

  

• Molds and mildews. Tiny fungus spores that can become airborne. These often thrive outdoors in soil, vegetation and rotting wood. They can be found indoors as well, especially in damp areas.
  
  

  

• Dust mites. Microscopic insects that inhabit household dust. They are often found indoors where there is less air circulation and on surfaces like carpets, bedding and stuffed animals.
  
  
• Animal dander. Tiny scales or particles from an animal’s skin. Indoors, these very often come from a pet (e.g., dog, cat) but can also come from pests (e.g., mice).
  
  
• Cockroach debris. Droppings, eggs, saliva and small pieces of outer shell of cockroaches. These can be found indoors, even if an infestation of live pests is not currently a problem.
  
  
• Tobacco smoke. Smoke from cigarettes, pipes, cigars or other tobacco products. These can cause complications in people with respiratory problems, such as asthma.

Most traditional types of air conditioner filters are incapable of removing tiny particles from the air. Instead, the air conditioners only help to distribute airborne allergens and particles throughout the house via the air ducts.

Some research suggests that HEPA filters provide only minimal relief in clean and well-ventilated environments. Other research suggests that HEPA vacuums are no more likely than non-HEPA vacuums to reduce a person’s exposure to dust mites. Individuals who are interested in purchasing a HEPA product should discuss the effectiveness of the appliance with their physician.

It should be noted that HEPA filters are not the only factor in preventing allergy and asthma symptoms. Although they can help reduce a person’s exposure to allergens and asthma triggers, there are many other steps a person should take to control these particles. For instance, individuals who are sensitive to tobacco smoke should not allow smoking in the home, regardless of how effective their HEPA filter is at removing the components of smoke from the air.

It should also be noted that HEPA filters are not effective at removing some heavier types of particles (e.g., intact pollen, heavy house dust) from the air. This is because these types of particles settle quickly once disturbed and are not suspended in the air long enough.

Because there are many allergens and irritants resting on the surfaces of the home, it is also important to clean areas such as countertops, drapery and furniture frequently. 

There are two different types of air filters currently available that use the name “HEPA.” Though both are good air filters, only one is truly capable of removing almost all types of allergens from the air. These two filters are:

• True HEPA. To qualify as a true HEPA filter, a filter must be able to remove 99.97 percent of all particles that are 0.3 microns in diameter or larger from the air. This means the filter can remove nearly all types of airborne allergens (e.g., pollen, mold) and many troublesome particles (e.g., tobacco smoke). True HEPA filters generally allow less airflow and cost more than other types of filters.
  
  
• HEPA-type. These filters often use the same kind of technology as a True HEPA, and are generally considered very good filters. However, their actual efficiency may be only 55 percent or less of particles that are 0.3 microns in diameter or larger. These filters are often much cheaper than True HEPA filters, and have a higher airflow.

There are several other types of air filters currently available for sale in the United States. Generally, HEPA filters – a type of mechanical filter – are considered the best. There are five basic types of air filters:

• Mechanical filters. These filters force air through a screen that is able to trap particles such as pollen, dander and dust mites. These filters are divided up into three different types:
  
  
• • Flat filters. Use a flat or panel filter often coated to increase particle adhesion.
    
    
  • Pleated filters. Use a filter that has been folded over to provide greater density.
    
    
  • HEPA filters. Use very fine filters to remove any particles larger than 0.3 microns.
    
    
• Electronic filters. These types of filters use electrical charges to attract and deposit particles. Many types of electronic filters use collecting places to capture the particles within the system.
  
  
• Hybrid filters. These filters use the technology of both mechanical and electronic filters to remove particles from the air.
  
  
• Gas phase filters. These filters are used to remove odors and non-particulate pollution from the air (e.g., cooking gas, gas from paints, perfumes), but do not remove any allergens from the air.
  
  
• Ozone generators. These devices produce ozone to supposedly clean the air. Neither the U.S. Environmental Protection Agency nor the American Lung Association recommends the use of these devices because high concentrations of ozone can be damaging to the lungs.

HEPA vacuum cleaners are also available. These use the HEPA filter technology to pick up particles such as dust, mold and pollen. Unlike conventional vacuum cleaners, HEPA vacuum cleaners are capable of capturing the particles and do not allow them to circulate back into the air.

HEPA air filters remove most allergens and troublesome particles from the air, making conditions better for people with a variety of conditions, including:

• Allergic rhinitis. Inflammation of the mucous membranes that line the nose due to an allergic reaction.
  
  

  

• Allergic conjunctivitis. Inflammation of the tissue lining the inside of the eyelid due to an allergic reaction.

  

• Asthma. Condition in which the airways become blocked or narrowed causing breathing difficulties. Many types of asthma attacks are triggered by particles in the air (e.g., dust, mold).
  
  

  

• Allergic sinusitis. Inflammation of the lining of the sinus cavities due to an allergic reaction leading to runny nose, headache and nasal congestion.
  
  

  

• Allergic bronchitis. Inflammation of the lung’s airways (bronchial tubes) that is brought on by an allergic reaction, usually to airborne allergens.
  
  
• Animal allergies. Allergic reactions caused by proteins found on the skin, hair and dried saliva (dander) of pets (e.g., cat, dog, bird) or other animals. HEPA filters are able to remove dander from the air, improving this condition.
  
  
• Airborne allergies. Allergic reactions caused by airborne particles such as pollen, mold, dust and other allergens.

HEPA filters need to be cleaned regularly. Both HEPA room air purifiers and HEPA vacuum cleaners need to have their filters changed (or cleaned on some types) when they are dirty to continue to function properly. How often they need to be changed depends on several different factors, such as:

• How often the filter is used. Filters that are used frequently will need to be changed (or cleaned) more often than those that are used only occasionally.
  
  
• Whether a prefilter is used. A prefilter is available on some models and works to stop many of the larger particles from reaching the main HEPA filter. The prefilters can often be cleaned and should allow the main HEPA filter to last longer.
  
  
• The kind of environment in which the filter is being used. Environments with molds, pets or tobacco smokers will usually require filter changes more often.

Individuals with allergies or asthma who use a HEPA vacuum cleaner should take care when cleaning the filter. It is a good idea to change the bag when it is about one-half full, and to change the bag outdoors, if possible. This keeps the dust that inevitably escapes from the bag during changing out of the house. Individuals who are highly allergic should wear a mask during the changing procedure.

In addition, studies have shown that vacuuming with a HEPA vacuum cleaner can temporarily increase the amount of allergens in the air as the cleaning process stirs up particles. It is a good idea for allergic individuals to wear a mask when cleaning or, if possible, have someone else clean for them.

Because HEPA filters are generally considered expensive items, they may not be a good solution for everyone. People shopping for a filter should investigate the many different brands available and take into account the cost of replacing old filters (on models where it is necessary). HEPA vacuums are more expensive than traditional vacuum cleaners, though they are often cheaper than HEPA air units.

Some good questions to ask when shopping for a HEPA filter are:

• Is this filter considered a True HEPA filter? How efficient is it in relation to a True HEPA filter?
  
  
• What is involved with changing the filter? Does it need to be replaced or just cleaned? How expensive is a filter?
  
  
• What kind of substances is the filter capable of removing from the air? What types of particles is it unable to remove?
  
  
• What size room is the filter capable of cleaning? How quickly can it filter all the air in a room?

Anyone interested in purchasing a filter should also be sure to:

• Listen to the filter while operating to see if it is too noisy
  
  
• Measure the square footage of the room (or space) where the filter will be used
  
  
• Ask about the filter’s ability to remove specific allergens (depending on the buyer’s own allergies)
  
  
• Watch this “Stuff We Love” video on a design-friendly HEPA filter
  
  

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to HEPA filters:

1. Would I benefit from the use of a HEPA filter or vacuum?
   
   
2. Can you explain to me how a HEPA filter works?
   
   
3. Other types of filter are less expensive. Would I benefit at all from using one of those?
   
   
4. Where can I purchase a HEPA filter or vacuum?
   
   
5. Can you recommend some effective models?
   
   
6. What should I consider when choosing a HEPA product?
   
   
7. Does it matter if I choose a HEPA-type filter or a True HEPA filter?
   
   
8. I have been using a HEPA filter for several months, why are my indoor allergies still acting up?