AIR contamination is a concern for ventilation engineers when it causes problems for building occupants. Engineers need to understand the vocabulary used by the air sampling and building air cleaning industry. This chapter focuses on the types and levels of air contaminants that might enter ventilation systems or be found as indoor contaminants. Industrial contaminants are included only for special cases. Because it is not a building air concern@ the effects of refrigerants on the atmosphere are not included in this chapter; see Chapter 19 for discussion of this topic.