Actually ordinary machines such as copier produces hefty amount of ozone too, that why you smell a sweet smell when the machine is operating. In fact, most hotels toilets uses even more ozone to keep the toilet bad smell away.
Nevertheless it is always good to err on the safe side. Most reefers will off their ozone after sweet the smell.
Ozone Generators in Singapore:
http://singapore.locanto.sg/ID_132335049/MINIATURE-PROFESSIONAL-OZONE-GENERATOR-TO-KILL-ODORS-MOLD-GERMS.html
http://www.eurovent.com.sg/
Health issues on Ozone:
http://www.epa.gov/apti/ozonehealth/population.html
http://www.ccohs.ca/oshanswers/chemicals/chem_profiles/ozone/health_ozo.html
http://www.epa.gov/glo/health.html
http://www.ccohs.ca/oshanswers/chemicals/chem_profiles/ozone/health_ozo.html
Personally to be, I feel that using Ozone is not dangerous if we know the risks to prevent overdose.