are energy efficient. HEPA technology is used in portable air purifiers as it removes common airborne allergens. The US Department of Energy has requirements manufacturers must pass to meet HEPA requirements. The HEPA specification requires removal of at least 99.97% of 0.3 micrometers airborne pollutants. Products that claim to be "HEPA-type", "HEPA-like", or "99% HEPA" do not satisfy these requirements and may not have been tested in independent laboratories. Air purifiers may be rated on a variety of factors, including Clean Air Delivery Rate (which determines how well air has been purified); efficient area coverage; air changes per hour; energy usage; and the cost of the replacement filters. Two other important factors to consider are the length that the filters are expected to last (measured in months or years) and the noise produced (measured in decibels) by the various settings that the purifier runs on. This information is available from most manufacturers. Potential ozone hazards As with other health-related appliances, there is controversy surrounding the claims of certain companies, especially involving ionic air purifiers. Many air purifiers generate some ozone, an energetic allotrope of three oxygen atoms, and in the presence of humidity, small amounts of NOx. Because of the nature of the ionization process, ionic air purifiers tend to generate the most ozone. This is a serious concern because ozone is a criteria air pollutant regulated by health-related US federal and state standards. In a controlled experiment, in many cases, ozone concentrations were well in excess of public and/or industrial safety levels established by US Environmental Protection Agency, particularly in poorly ventilated rooms. Ozone can damage the lungs, causing chest pain, coughing, shortness of breath and throat irritation. It can also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections—even in healthy people. People who have asthma and allergy are most prone to the adverse effects of high levels of ozone. For example, increasing ozone concentrations to unsafe levels can increase the risk of asthma attacks. Due to the below average performance and potential health risks, Consumer Reports has advised against using ozone producing air purifiers. IQAir, the educational partner of the American Lung Association, has been a leading industry voice against ozone-producing air cleaning technology. Ozone generators used for shock treatments (unoccupied rooms) which are needed by smoke, mold, and odor remediation contractors as well as crime scene cleanup