Service manual

STP 11-25R13-SM-TG
T - 17
Section IV. MISCELLANEOUS HAZARDS
T-23. Mercury.
Very small amounts of mercury can vaporize and result in dangerous amounts of mercury vapor in
the air. A time-weighted average (TWA) of 0.05 milligrams per cubic meter of air has been established by
the American Conference of Government Industrial Hygienists (ACGIH). This is a TWA concentration of
mercury in the air that would not be expected to cause illness in nearly all workers breathing this amount
40 hours a week for his working life. If 25 milligrams (mg) of mercury (a droplet of mercury the size of a
pinhead weighs about 1,000 milligrams) were vaporized in 100 cubic meters of air, the mercury
concentration of one-half of the TWA would be attained. Supervisory personnel should conduct daily
visual contamination surveys. Any area showing contamination should be decontaminated. For those
facilities having ready access to a mercury vapor meter, mercury surface contamination surveys should
be conducted monthly. For other facilities, qualified personnel should conduct mercury surface
contamination surveys at least annually. Contaminated areas should be marked for decontamination and
resurveyed after decontamination to ensure the cleaning procedures were effective. Work surfaces and
floors where mercury is likely to be spilled should be smooth and free of cracks. Mercury not in use will
be stored in closed containers to minimize spillage and contamination. Personnel subject to handling or
working around mercury must exercise the following precautions:
a. To minimize breakage, be careful when handling instruments containing mercury. The mercury
vapor detector will be placed in operation before working with open containers of cold or hot mercury.
b. Be extremely careful when working around hot mercury. Hot mercury will produce considerably
more mercury vapor than cold mercury.
c. Eating, drinking, or smoking will not be permitted in the mercury work areas. Wash hands after
leaving mercury work areas.
d. Be sure that mercury does not adhere to clothing. Personnel required to handle mercury shall
wear three-quarter length smocks (IL Federal supply catalog, C-8405/25-IL-A, FSC:8415).
e. Practice good housekeeping at all times to be sure that no small droplets of mercury are
allowed to remain undetected. To minimize contamination of the work area, work (e.g., cleaning or
changing mercury, doping terminals, etc.) shall be accomplished over a half-full pan of water with
sidewalls at least 2 inches high, unless prohibited by size, configuration, or handling equipment. This
method confines the mercury and minimizes the escape of vapors.
f. Be sure that general ventilation, providing at least 10 room air exchanges per hour, is provided
in all areas where mercury is used. An exhaust hood is required wherever mercury or an instrument
containing mercury is heated. Such hoods shall have an average air velocity of 150 fpm through the fully
open faces. Bypass openings shall be provided to maintain proper hood and room balances. The
variation in air velocity through the face shall not exceed +/-20 percent. Fans for exhaust hoods shall be
located at the discharge point of the system.
g. When pumps are used to pull a vacuum on a mercury reservoir (for example, nitrometers and
some gas analysis equipment), the effluent air from the pumps should be discharged directly outdoors
through an adequate exhaust vent and a water trap should be placed between the mercury reservoir and
the pump. Otherwise, the pumps may become heavily contaminated with mercury and become a serious
source of contamination if the pumps are used for other laboratory purposes.