Is ozone dangerous to breathe

Recent research indicates that it is.

Breathing ozone gas causes inflammation in the airway. If someone has Asthma, this is life threatening.

Ozone dissolved in the bloodstream has been linked to converting "good cholesterol" into "bad cholesterol", which can harden arteries and makes the heart pump harder. If you have a bad heart, or a problem with cholesterol, this can be life threatening.

Beyond this, white blood cells make ozone (and other powerful oxidants) when fighting off infection. Waterfalls and lightning make ozone. Ozone is something organisms have been required to deal with, in some quantities, as long as there has been oxygen and life. Some ozone must be present. What is "normal" is the hard question, and probably depends on locale, time of year, and more.