Class I Atmospheres (gas or vapors)

Group A
actylene

Group B
acrolein (inhibited)2
butadiene1
ethylene oxide2
hydrogen
manufactured gases containing more than 30% hydrogen (by volume)
propylene oxide2

Group C
acetaldehyde
allyl alcohol
n-butyraldehyde
carbon monoxide
crotonaldehyde
cyclopropane
diethyl ether
diethlamine
epichlorohydrin
ethlene
ethyenimine
hydrogen sulfide
morpholine
2-nitropropane
tetrahydrofuran
unsymmetrical dimethyl hydrazine
   (UDMH 1,1-dimethyl hydrazine)

Group D
acetuc acid (glacial)
acetone
acrylonitrile
ammonia3
benezene
butane
1-butanol (butyl alcohol)
2-butanol (secondary butyl alcohol)
n-butyl acetate
isobutyl acetate
sec-butyl alcohol
di-isobutylene
ethan
ethanol ( ethyl alcohol)
ethyl acetate
ethyl acrylate (inhibited)
ethylene diamine (anhydrius)
ethylene dicloride
gasoline
heptanes
hexanes
isoprene
isopropyl ether
mesityl oxide
methane (natural gas)
methanol (methyl alcohol)
3-methyl-1-butanol (isoamyl alcohol)
methyl ethyl ketone
Methyl isobutyl katone
2-methyl-1-propanol (isobutyl alcohol)
2-methyl-2-propanol (tertianry butyl alcohol)
petroleum naptha4
pyridine
octanes
pentaces
1-petanol (amyl alcohol)
propane
1-propanol (propyl alcohol)
2-propanol (isopropyl alcohol)
propylene
styrene
toulene
vinyl acetate
vinyl chloride
xylenes

Division 1
Hazardous concentrations of flammable gases or vapors exist continuously, intermittently, or periodically: under normal operating conditions; because of repair or maintenance operations; due to breakdown of equipment which might release flammable gases or vapors. 

Division 2
1. Flammable liquids or gases are handled but are confined in closed containers and can escape only in case of accidental breakdown.
2. Concentrations of gases are normally prevented by positive mechanical ventilation but might become hazardous through failure of ventilation system. 
3. Locations adjacent to Class I, Division 1 locations in which hazardous concentrations are prevented by positive-pressure ventilation from a source of clean air with safeguards provide against ventilation failure.