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Hydrogen Chloride (HCl)

Exposure Effects
Existing Guidelines
Volcanic Examples and Incidents


Hydrogen Chloride (HCl) is a colourless gas with an irritating pungent odour detectable at ~0.8 ppm. It is very soluble in water (673 g L-1 at 30°C) (Gangolli, 1999) and reacts in moist air to form a mist. It is non-flammable in air and has a density of 1.49 g L-1 at 25°C and 1 atm (Lide, 2003), 1.2 times that of ambient air. Typical concentration ranges of HCl in dilute volcanic plumes are 0.1-3 ppm, compared to the tropospheric background of 0.000001-0.001 ppm, and the gas has a residence time in the lower atmosphere of approximately 4 days (Brimblecombe, 1996; Oppenheimer et al., 1998).

Exposure Effects

The high solubility of HCl means that exposure will usually be to a mixture of gas and aerosol. HCl is highly irritating to the eyes and the mucous membranes. On contact with the skin it can cause skin inflammation or burns (NIOSH, 1981). Prolonged, elevated exposures to HCl mist can give rise to erosion of teeth and may cause ulceration of the inner nose. Inhalation can cause choking, coughing, and swelling in the respiratory system depending upon the severity of exposure. Concentration thresholds for health effects are outlined in the table.

Health effects of respiratory exposure to hydrogen chloride
(Baxter, 2000; Faivre-Pierret and Le Guern, 1983 and references therein; NIOSH, 1981; Sax and Lewis, 1989).

Exposure limits (ppm) Health Effects
5< Coughing
35 Throat irritation occurs after only a short time
35< Severe breathing difficulties and skin inflammation or burns
10-50 Maximum level that can be sustained for several hours
100< Swelling of the lungs and often throat spasm
50-1000 Maximum possible exposure = 1 hour
1000-2000 Very dangerous even for a very short exposure

Existing Guidelines

Only occupational guidelines exist for gaseous HCl and these are given in the table.

Occupational Guidelines for HCl

Country/ Institution Level (ppm) Level µg m-3 Averaging Period Guideline Type Date of Implemen- 
Relevant Law Notes Ref.
EU 5 8000 8 hour TWA OEL   Commission Directive 96/94   a
10 15000 STEL OEL   Commission Directive 96/94   a
UK 5 8000 15 min MEL   ILV   b
1 2000 8 hour TWA MEL   ILV   b
USA 5 7000 8 hour TWA PEL   OSHA Regulations (Standards - 29 CFR) 1 c
5 7000 ceiling REL 2003 NIOSH   d
3   1 hour ERPG-1 1998 Emergency Response Planning Guideline   e
20   1 hour ERPG-2 1998 Emergency Response Planning Guideline   e
150   1 hour ERPG-3 1998 Emergency Response Planning Guideline   e
  1. ppm by volume at 25ºC and 760 torr.
  1. http://europa.eu.int/comm/employment_social/health_safety/docs/oels_en.pdf
  2. HSE, 2002. Occupational Exposure Limits 2002. HSE Books, Sudbury
  3. OSHA Standards Website
  4. NIOSH Pocket Guide to Chemical Hazards (NPG) http://www.cdc.gov/niosh/npg/npg.html
  5. AIHA Emergency Response Planning Guidelines Committee, 2002. Emergency Response Planning Guidelines 2002 Complete Set, American Industrial Hygiene Association, Fairfax.

Volcanic Examples and Incidents

Measurements of fluxes of HCl are much more common than concentrations and there are no known incidents involving high concentrations of HCl. However, concentrations of HCl close to vents can exceed occupational standards:

  • Yasur, Vanuatu: Gas samples collected at the crater rim in September 1988 contained HCl concentrations between 3 and 9 ppm (SEAN 13:12).
  • Kilauea, Hawaii: Gas concentrations at Pu'u O'o vent varied from 0.10 ppm to 19.15 ppm in July 2003 (C. Witham unpublished data). HCl levels in dense LAZE plumes formed where lava from Kilauea reached the sea in March 1990 averaged 7.1 ppm. (Kullman et al., 1994), exceeding occupational levels. These concentrations decreased rapidly with distance from the source. (Note: The primary chlorine source for LAZE is seawater, not volcanogenic gases). Measurements in 2004 yielded HCl concentrations in the 3-20 ppm range just above Pu`u O`o on the east rift zone, and 1.5-4.5 ppm near the source of the ocean entry plume. These likely represent minimum values. (HVO, unpublished data).
  • Popocatepetl, Mexico: Near-vent concentrations of HCl in February 1997 were ~1 ppm (2000 µg m-3) (Goff et al., 1998), which is equal to the UK 8-hour average occupational limit.
  • Villarrica, Chile: HCl concentrations at Villarrica lava lake occasionally exceed the USA occupational guideline of 5 ppm (Witter and Delmelle, 2004).
  • Masaya, Nicaragua: In March 1999, maximum plume concentrations averaged over the Masaya crater were >23 ppm (Horrocks et al., 1999), greatly exceeding nearly all guidelines. Measurements made 14 km downwind of the vent in March 1998, were 0.3 ppm (Horrocks, 2001). Maximum HCl concentrations at the crater-rim in May 2001 were 0.902 ppm (1300 µg m-3) (Allen et al., 2002), which is just below the UK 8-hour occupational guideline.


Allen, A.G., Oppenheimer, C., Ferm, M., Baxter, P.J., Horrocks, L.A., Galle, B., McGonigle, A.J.S. and Duffell, H.J., 2002. Primary sulfate aerosol and associated emissions from Masaya Volcano, Nicaragua. Journal of Geophysical Research, 107(D23).

Baxter, P.J., 2000. Gases. In: P.J. Baxter, P.H. Adams, T.-C. Aw, A. Cockcroft and J.M. Harrington (Editors), Hunter's Diseases of Occupations. Arnold, London, pp. 123-178.

Brimblecombe, P., 1996. Air Composition and Chemistry. Cambridge University Press, Cambridge.

Faive-Pierret, R. and Le Guern, F., 1983. Health risks linked with inhalation of volcanic gases and aerosols. In: H. Tazieff and J.C. Sabroux (Editors), Forecasting Volcanic Events. Elsevier Science Publishers B.V., Amsterdam, pp. 69-81.

Gangolli, S. (Ed.), 1999. The Dictionary of Substances and their Effects, 2nd edn. The Royal Society of Chemistry. Cambridge.

Goff, F., Janik, C.J., Delgado, H., Werner, C., Counce, D., Stimac, J.A., Siebe, C., Love, S.P., Williams, S.N., Fischer, T. and Johnson, L., 1998. Geochemical surveillance of magmatic volatiles at Popocatpetl Volcano, Mexico. Geological Society of America Bulletin, 110(6): 695-710.

Kullman, G.J., Jones, W.G., Cornwell, R.J. and Parker, J.E., 1994. Characterization of air contaminants formed by the interaction of lava and sea water. Environmental Health Perspectives, 102(5):478–482.

Lide, D.R. (Ed.), 2003. CRC Handbook of Chemistry and Physics, 84th edn. CRC Press. Boca Raton, Florida.

National Institute for Occupational Safety and Health (NIOSH), 1981. Occupational Health Guidelines for Chemical Hazards, DHHS (NIOSH) Publication No. 81-123. http://www.cdc.gov/niosh/81-123.html.

Oppenheimer, C., Francis, P., Burton, M., Maciejewski, A.J.H. and Boardman, L., 1998. Remote measurement of volcanic gases by Fourier transform infrared spectroscopy. Applied Physics B, 67: 505-515.

Sax, N.I. and Lewis, R.J., Sr., 1989. Dangerous Properties of Industrial Materials, 7th edn. Van Nostrand Reinhold. New York.

Smithsonian Institution, 1988. Yasur. Scientific Event Alert Network (SEAN) Bulletin, v. 13, no. 12.

Witter, J.B. and Delmelle, P., 2004. Acid gas hazards in the crater of Villarrica volcano (Chile). Revista Geologica de Chile, 31(2): 273-277.