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Air Purification > Sizing & Safety
  Sizing Overview
  Sizing Calculations
  Safety Considerations
Aside from selecting the technology and the brand for an air purifier, it is also important to make sure that the unit is appropriately sized for its intended usage. An air purifier that is too weak for the space will not rid the air of impurities to the full extent while one that is too strong will waste energy. In addition to sizing, safety is also an important consideration, as few things have as profound an impact on human health as the air we breathe.
Sizing Overview
Sizing is an even more important consideration with respect to air purifier selection than energy efficiency. A properly sized air purifier will clean all of the air in a particular indoor space. To assure that this happens, it is necessary to ensure that the air purifier can handle a volume of air that is commensurate with the size of the interior space where it is placed. Even the best air purification technology will not be useful if it is able to process only a portion of the air that is actually in the room and being breathed by the residents. The goal is to clean all of the indoor air in a particular set of living spaces.

To understand the importance of sizing the air purifier, consider the following analogy. Imagine that instead of air, the goal is to clean a certain quantity of soiled clothes, and instead of an air purifier the equipment being used is a washing machine. The washing machine may be the most powerful in the world, but since it is only possible to run a single load at one time, the limiting factor is not the washing machine’s power but rather its capacity. If only half of the soiled clothes actually fit inside the washing machine, then only half of the clothes will be cleaned while the other half remains dirty. Certainly, a second load can be run, but imagine a scenario where during the time the second load is being run, the first load of newly clean clothes is soiled again. In such a situation half the clothes will always remain dirty.

Applying this analogy to an air purifier, if the unit’s capacity is insufficient for the room where it is placed, then a portion of the air will remain dirty. Put a different way, if there is a greater volume of air in the room than the air purifier can actually handle, then it is inevitable that some part of the air in the room will not get cleaned. Consequently, it is vitally important to select an air purification unit that is appropriately sized for the room.

Air purification is not a very well-regulated industry, but certain steps to provide common measures and guidelines have been undertaken by the Association of Home Appliance Manufacturers (AHAM). This is a trade group comprised of many of the world's largest home appliance companies. The AHAM has put forward a standard known as the Clean Air Delivery Rate (CADR) for the specific purpose of measuring air purifier performance. In designing the CADR, the AHAM based it on a recommended standard for measuring particulate removal that was adopted in 1988 by the American National Standards Institute (ANSI). In turn, the ANSI standard was peer reviewed prior to adoption and accepted by the scientific and public-health communities as valid.
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Sizing Calculations 
The CADR is a measure of the amount of air that a purifier is able to clean during one minute of operation. The CADR number is expressed in cubic feet per minute (CFM). Every quality air purifier will have an AHAM seal which expresses three CADR values for the unit - one for tobacco smoke, one for pollen, and one for dust. Higher CADR values mean that the unit is able to clean a greater amount of air within a given period of time.

The AHAM recommends the so-called "2/3 rule" as a rule of thumb. They suggest taking the square footage of the room where the air purifier will be placed and checking to see that the unit’s CADR for tobacco smoke is at least 2/3 of that value. If, for example, the intention is to place the air purifier in a bedroom measuring 15 feet by 15 feet, then the calculation is as follows: 15 times 15 gives the square footage of the room to be 225 feet; this means that the CADR for tobacco smoke on the unit should be no less than 2/3 of 225, which is 150.

Another way to approximate the necessary CADR level is to consider the desired number of Air Changes per Hour (ACH) for the room. An air change refers to the amount of time it takes for all of the air in the room to cycle through the air purifier once. Most air purification experts and allergists recommend at least 2 ACH, although some experts recommend a number as high as 6.

To calculate how many air changes a particular air purifier will be able to perform in an hour, first calculate the cubic feet of air that are present in that room. To do this, simply take the square footage of the room and multiply it by the height of the room. Using our example of a 225 square foot bedroom, let us suppose that the bedroom has 10 foot ceilings. In that case, the volume of air in the room is 225 multiplied by 10, which is 2250 cubic feet. An air purifier that has a CADR of 150 cubic feet per minute would then require: 2250 cubic feet (room volume) divided by 150 (volume cleaned per minute), or 15 minutes to complete one full air change. That means that in an hour, the air purifier could complete 4 full air changes in our sample room, giving it a rating of 4 ACH for that room. That is well above the recommended minimum level of 2 ACH.

The third and final way to approximate unit size is by following the manufacturer's advertised guideline. Many air purifiers are explicitly marketed as being able to handle rooms up to a particular size. However, this is the least dependable way to select an air purifier because there is no single consistent methodology among manufacturers for such a measurement. As a result, most manufacturers exaggerate their units' capabilities with respect to room size. You are better off checking for the AHAM seal and using the official CADR rating instead, as this is a more objective measure of the unit's capacity.
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Safety Considerations
Air purifiers act directly upon the air which people breathe, which makes safety a critical consideration in selecting a product. This is particularly important in light of the fact that air purification is a largely unregulated industry in which certain manufacturers have had a history of making false claims and selling products which were subsequently shown to have adverse health effects.

Many of the critical health issues in the air purification space have revolved around the production of ozone, whether directly in the case of ozone generators or indirectly in the case of many ionizers and electrostatic precipitators. To understand why there has been so much controversy regarding these products it is important to take a step back and consider the health effects of ozone.

Also known as trioxygen, ozone is a triatomic molecule consisting of three oxygen atoms. This molecule is much less stable than the more common diatomic allotrope which consists of two oxygen atoms. The ozone layer in the upper atmosphere is beneficial in that it prevents damaging electromagnetic radiation from reaching the Earth’s surface. However, ozone found in the lower atmosphere and near the surface is actually an air pollutant which exhibits harmful effects on the respiratory systems of animals and the lifespans of certain plant forms.

Exposure to ozone has been linked to a host of health ailments, including asthma, bronchitis, heart attack, and various other cardiac and pulmonary illnesses. A study of nearly half a million people living in the United States showed a significant statistical correlation between ozone exposure and incidence of respiratory illness over a two decade period. The study showed that people living in cities with high ozone levels such as Houston or Los Angeles had a 30% higher risk of dying from lung disease.

Both the EPA and the European governing bodies have passed legislation prescribing a maximum allowed level of ozone and requiring municipalities with higher levels to take action to reduce the amount of ozone in their respective areas. These regulations refer to ozone concentrations present outdoors.

Ozone can also be present in indoor air pollution, partly as a result of the operation of electronic equipment such as photocopiers and other common appliances. Obviously, the usage of ozone generators or ionizers which release ozone as a byproduct also acts to directly increase the amount of ozone present indoors.

When ozone is inhaled, it reacts with the lining of the lungs to form certain metabolites which are believed to lead to the build-up of atherosclerotic plaques, which represent a type of heart disease. Even at low concentrations, ozone can affect both vision and, as indicated, the respiratory system. Studies suggest that severe and permanent lung injury and even death could follow from very short-term exposures to relatively low concentrations of ozone.

Clearly, ozone is highly dangerous and when information regarding the danger of ozone generators and ionizers first began to appear, manufacturers of these air purifiers began to scramble and put out marketing suggesting that while it was true that their products created ozone, the amount of ozone was safe for human use. In most cases, this was simply more disinformation to prevent the loss of customers. It was truly saddening to see that these companies were willing to sacrifice people’s health at the altar of higher profits.

Given the numerous safe and more effective air purification alternatives, there is simply no good reason to purchase ozone producing air purifiers. Before buying a product, consumers are strongly advised to perform due diligence and ensure that the particular unit does not produce either ozone or any other hazardous chemical byproduct.
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