### Question
How is the "class I substance" defined?


### Answer
The term **class I substance** is defined in *42 U.S.C. § 7671* as follows -

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

#### Citation:
  - *42 U.S.C. § 7671*
  - Heading: **CHAPTER 83-ENERGY EXTENSION SERVICE > SUBCHAPTER VI-STRATOSPHERIC OZONE PROTECTION**

