Carbon Dioxide - OSHA's former limit for carbon dioxide was 5000 ppm as an 8-hour TWA. The ACGIH has a 5000-ppm TLV-TWA with a 30,000-ppm TLV-STEL, and these were the limits proposed. NIOSH has a TWA REL of 10,000 ppm with a 10-minute 30,000-ppm ceiling limit; however, NIOSH (Ex. 8-47, Table N1) concurred that the proposed limits were appropriate. After carefully reviewing the record evidence submitted in response to OSHA's proposal for carbon dioxide, the Agency has determined that exposure limits of 10,000 ppm (8-hour TWA) and 30,000 ppm (15-minute STEL) are appropriate. Carbon dioxide is a colorless, odorless, noncombustible gas.