sentences:  SMACs guide chemical exposures during normal as well as emergency operations aboard spacecraft.Short-term SMACs refer to concentrations of airborne substances such as gas and vapour that will not comprise the performance of specific tasks by astronauts during emergency conditions or cause serious toxic effects.Long-term SMACs are intended to avoid adverse health effects and to prevent any noticeable changes in the crew's performance under continuous exposure to chemicals for as long as 180 days.