Mercury threshold limits values

H. Roels, J.P. Gennart, R. Lauwerys, J.P. Bucher, J. Malchaire, and S. Bernard, Surveillance of workers exposed to mercury vapor Validation of a previously proposed biological threshold limit value for mercury concentration in urine. Am. J. Ind. Med. 7 45, 1985. 

Ambient mercury vapor concentrations of 100 pg/ m or higher have been measured during chloralkah production and mercury mining [18]. Adverse health effects were common sequelae from such exposures. During recent years, most countries have reduced mercury s occupational threshold limit value to 50... 

At least 11 metals are biologically toxic, as shown in Table I along with their threshold limit values (1,2,3) arsenic, beryllium, bismuth, cadmium, mercury, nickel, lead, antimony, selenium, tin, and vanadium. For these and other metals, the biological toxicity for long-term, low-level exposures is not known. Other metals such as manganese and iron are... 

There is no known biochemical reaction in organisms that applies Hg as an essential element. Mercury is the only metal which is a liquid at ordinary temperatures. The boiling point of this metal is 357 °C. This temperature is relatively low for metals and its vapor pressure is significant even at room temperature. The threshold limit value (TLV) of elemental mercury is 0.05 mg/m- a value that is less than the equilibrium vapor pressure at ambient temperatures. However, in the mercury miners in Sicily, where the mercury occurs in shales, the miners are exposed to elemental mercury vapor, which content in the air may reach toxic levels of about 5 mg/nr Another source of exposure in mines is the mercury-containing dust. 

NIOSH determined a Recommended Exposure Limif (REL), time-weighted average, of 50 pg Hg m for mercury vapor, and 100 pg m for aryl and inorganic mercury compounds (NIOSFl 1983). ACGIFl established the same concentrations as 8-hour time-weighted averages as Threshold Limit Values" (TLVs). 

Metallic or elemental mercury volatilizes to mercury vapor at ambient air temperatures, and most human exposure is by way of inhalation. The saturated vapor pressure at 20.0°C is 13.2mg/m. This value far exceeds the threshold limited value (TLV) of 0.05 mg/m accordingly, mercury intoxication due to inhalation of the vapor readily occurs in various occupational and environmental situations. Mercury vapor readily diffuses across the alveolar membrane and is hpid soluble so that it has an affinity for the central nervous system and red blood cells. Metallic mercury, unlike mercury vapor, is only slowly absorbed by the G1 tract (0.01%) at a rate related to the vaporization of the elemental mercury and is of negligible toxicological significance. 

At room temperature (25°C) the equilibrium concentration of mercury vapour is about 20 mg/m or 200 times the threshold limit value of 0.1 mg/m recommended as a maximum atmospheric concentration for normal work schedule by the American Conf. of Governmental Hygienists. For information on the toxicity of mercury and safe handling of mercury the reader is referred to the manual of the 646 VA processor by Metrohm, Chapter 6.4, and references therein. 

Bimetallic strip thermometers are preferred for monitoring oven temperatures. Mercury thermometers should not be mounted through holes in the tops of ovens so that the bulb hangs into the oven. Should a mercury thermometer be broken in an oven of any type, the oven should be closed and turned off immediately, and it should remain closed until cool. All mercury should be removed from the cold oven with the use of appropriate cleaning equipment and procedures (see Chapter 5, section 5.C. 11.8) in order to avoid mercury exposure. After removal of all visible mercury, the heated oven should be monitored in a fume hood until the mercury vapor concentration drops below the threshold limit value (TLV). 

The results for each of the six plants are presented in this section of the paper and are identified by project plants 1-8. This section addresses those results relevant to occupational exposures (i.e. at or above the Action Level), environmental emissions (i.e. potential air emissions above background) and process equipment (i.e. locations with potential mercury accumulation). Occupational exposures were compared to the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) for inorganic mercury, 25 pg/scm (micrograms per standard cubic meter). The project sponsor adopted Action Level (AL) is 12.5 pg/scm. The AL is used as an early indicator for potential mercury exposure approaching the TLV. 

To verify this proposition to some extent, mercury fulminate was irradiated with a 75 mJ laser pulse, a value which is below the initiation energy threshold (for the used sample holder). The emission spectrum recorded 270 nsec after the start of the laser pulse does not reveal emission lines/bands or species different from those obtained after explosive irradiation [40], although some changes in the relative intensity of the emission lines were noticed. Although additional experiments to verify this proposal and to establish its limitations are needed, it is believed that the emission spectra of irradiated unconfined explosives give useful information about decomposition intermediates. This belief is supported by the observation of similar species in emission spectra recorded under various circumstances [14]. 

---