Page 18 - Confined Space Training - Student Manual 2021

Alternate Sources of Contaminants

It is possible that hazardous materials can enter a confined space from sources outside the space.
• A utility manhole situated in the vicinity of an automotive service station may contain
gasoline vapors that migrated from a leaking underground storage tank. While the
concentration of these vapors may be below the lower explosive limit, they could still pose
a potential inhalation hazard.
• An old landfill can produce methane and/or hydrogen sulfide, which could seep into a
confined space.
• Nearby gas lines or sewage lines also represent potential contaminants.
• Old industrial plants that may have utilized solvents in their processes could be a source
of contamination if some of those materials leaked into the ground.
• Vapors from cleaning solvents or fumes from welding can present a danger to confined
space workers.

Understanding Vapor Density

It is important to know the vapor density of gases and vapors that you may find in confined
spaces on your site. Air is the referenced standard for determining vapor density. Air has
a vapor density of 1. Gases with a vapor density of less than 1 will rise. Gases with a vapor
density greater than 1 will sink.

Common Atmospheres in Confined Spaces

• Air (Vapor Density=1.00)
o Distributed throughout the space
o May be displaced by other gases

• Oxygen O2 (Vapor Density=1.43) Methane
o Heavier than air
o Will collect in lower spaces Carbon
o Enhances combustion above 21% Monoxide
• Methane CH4 (Vapor Density=0.554)
o Lighter than air
o Will collect at upper spaces Air
o Flammable
• Carbon Monoxide CO (Vapor Density=0.968) Hydrogen
o Slightly lighter than air Sulfide
o Will collect in upper areas
o Flammable/Toxic Methane
• Hydrogen Sulfide H2S (Vapor Density=1.189)
o Heavier than air
o Will collect in lower areas
o Flammable/Toxic

Confined Space Training for Entrants, Attendants & Supervisors 18