Walk-in refrigerators (or "cold boxes") typically recirculate the chilled air in their interiors, so storing volatile materials in them can pose special hazards; any gases or vapors may concentrate inside over time.
Recently on campus, a walk-in refrigerator was used to store dry ice. The dry ice was stored in a standard dry ice storage locker, but the locker had been placed in the cold box to further reduce the rate of dry ice loss. The dry ice, of course, gave off carbon dioxide (CO2) gas as it sublimed, causing the refrigerator to build up CO2 levels of 12,000 parts per million. In comparison, outdoor air contains only about 400 ppm CO2, and OSHA's Permissible Exposure Limit for CO2 is 5000 ppm. Although no one was affected, the incident points out the need to keep volatiles out of walk-in refrigerators.