The first is Odor and the second is Toxicity. In both cases, there can be a serious health risk to the person who is exposed to the gas. An Odiferous gas is the condition where an organic material emits an objectionable or revolting odor. In rating an odiferous gas, you must consider not only the smell, but also whether exposure to the gas causes nasal irritation or any other medical problems. Materials that are hermetically sealed in containers do not have to meet the criteria because the odiferous gas is not exposed to breathing gases.
Toxicity is the condition where harmful or toxic gases encounter breathable gases.
JSC 20584 states that SMACs are guidance concentrations that are designed to protect personnel from the adverse health effects of being exposed to toxic gases. Long-term exposure is considered to be 7 to 180 days. No level of exposure for any SMAC is acceptable if it is great enough to degrade mission effectiveness. While NASA categorizes their toxicological groupings according to the affected organ and nature of the adverse effect, it is understood that most toxins have a wide range of effects that cross the groupings. Another area of concern is the use of polymeric materials in environments that support combustion.