The same chemical properties that allow high concentrations of ozone to react with organic material outside the body give it the ability to react with similar organic material that makes up the body, and potentially cause harmful health consequences. When inhaled, ozone can damage the lungs (see - "Ozone and Your Health"
. Relatively low amounts can cause chest pain, coughing, shortness of breath, and, throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections. People vary widely in their susceptibility to ozone. Healthy people, as well as those with respiratory difficulty, can experience breathing problems when exposed to ozone. Exercise during exposure to ozone causes a greater amount of ozone to be inhaled, and increases the risk of harmful respiratory effects. Recovery from the harmful effects can occur following short-term exposure to low levels of ozone, but health effects may become more damaging and recovery less certain at higher levels or from longer exposures (US EPA, 1996a, 1996b).
http://www.epa.gov/iaq/pubs/ozonegen.html
The FDA has also seized ozone generators. See, e.g.,
FDA declares war on ozone generators; seizes inventory
"The seized devices are potentially harmful to public health," said the agency's acting associate commissioner for regulatory affairs, Michael Chappell. "The agency will take action to protect the public from FDA-regulated products that are in violation of the law."
http://www.naturalnews.com/028958_ozone_generators_FDA.html