### Question
Can you define what class I substance means?


### Answer
**Class I substance** is defined as the following in *42 U.S.C. § 7671*:

> each of the substances listed as provided in section 7671a(a) of this title

#### Citation:
  - *42 U.S.C. § 7671*
  - Heading: **CHAPTER 85-AIR POLLUTION PREVENTION AND CONTROL > SUBCHAPTER VI-STRATOSPHERIC OZONE PROTECTION**

