Demystifying Hazmat Chemistry

STUDENT HANDOUT
DEMYSTIFYING HAZMAT CHEMISTRY
Vapor Pressure and Vapor Density
Approximately 90% of hazmat injuries are due
to inhalation. Obviously, this signals a need
to protect against exposure to gases. However,
responders must also be concerned about the
vapors produced by liquids (and, to some extent,
the vapors produced by solids). To determine
whether vapors can “get to us,” we need to look
at vapor pressure.
Vapor pressure is the force exerted by the vapors of a product against the atmosphere or the sides
of a container. If a liquid has a vapor pressure less than 10 mmHg (millimeters of mercury), the
vapors won’t travel much beyond the surface of the liquid. However, above 10 mmHg, the vapors
will begin to rise into the air. How far and how fast they travel depends largely on vapor pressure.
The chart below shows the vapor pressures of several products for comparison.
The Vapor Pressures of Various Products
Source: NIOSH Pocket Guide to Chemical Hazards
Vapor pressure is normally measured in millimeters of mercury (mmHg) at 68˚F (20˚C) at sea
level. (If measured at another temperature, that temperature should be identified.) If the vapor
pressure equals or exceeds 760 mmHg at sea level, the product is a gas in its normal state. The
closer the vapor pressure is to 760 mmHg, the more vapors a liquid produces and the more you
need to protect yourself against those vapors. (Vapor pressure may also be reported in pounds
per square inch or atmospheres. A vapor pressure of 760 mmHg is equivalent to 14.7 psi or 1
atmosphere.)
Vapor pressure is temperature-dependent. The greater the temperature, the higher the vapor
pressure. The following page contains a chart that shows the vapor pressure of water at various
temperatures. Notice how it builds from 17.5 mmHg at 68˚F (20˚C) to 760 mmHg at 212˚F
(100˚C).
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