It seems obvious that people buy household air purifiers to purify the air, whether in order to remove odors or ease allergies and asthma. In fact, a good number of air purifiers actually do the opposite: they pollute the air with ozone. Though you may have heard of ozone as a good thing, that always depends on whether it's high up in the ozone layer or down in the air you breathe. The EPA coined the phrase "good up high, bad nearby" to define which is which: in the upper atmosphere, ozone helps protect the planet from UV radiation; in the atmosphere that surrounds us, it can cause irritation and inflammation in the lining of your respiratory system. Elevated or repeated exposure can cause permanent lung damage and even death, leading the state of California to ban ozone-producing air purifiers from the market. But why do these devices produce ozone in the first place? Manufacturers claim that because ozone is a highly reactive molecule, it easily neutralizes contaminants in the air and leaves only oxygen, water, and carbon dioxide behind. The science shows that the byproducts of these chemical reactions are not always so harmless, sometimes producing toxic chemicals such as aldehydes and formic acid. Even when they don't produce toxic chemicals, the reactions can take months or years, not minutes. Beyond that, research shows that ozone is ineffective at cleaning the air when it's at levels that don't harm human health. So what's a consumer to do for fresh, clean air? Consumer Reports suggests you stay away from air purifiers that use electrostatic or ozone-generating technology, and go for devices that rely on HEPA filters instead. We've collected some awesome videos on this topic. Watch them now to learn more.