of focal nodular lesions in the upper lung, a site that distinguishes this disease from asbestosis. In both diseases, lung function may not initially be markedly affected, but with continual exposure the progression of lesions reduces the original pliable lung to stiff fibrotic tissues, and occlusion of the air sacs compromises the transmission of essential gasesby the respiratory system. Silicosis was identified as a major health concern in the 1930s, but few new cases have appeared in recent years because of increased attention to hazardous workplace environments. The identification of silica in volcanic emissions has caused some recent concern (Horwell et al., 2003).

There is a large, and expanding, inventory of gaseous-phase aerosols that can potentially cause harm to the human respiratory system. These can be natural or anthropogenically generated and are found outside and indoors (McElroy, 2002). The primary natural outdoor pollutants are hydrocarbons from plant respiration, biogenic gases such as methane (CH₄) and hydrogen sulfide (H₂S), and volcanic gases such as sulfur dioxide (SO₂), H₂S, carbon monoxide (CO), and carbon dioxide (CO₂). Radon, a naturally generated gas, is a major indoor air pollutant.

Volcanic eruptions produce enormous quantities of gas that, in some situations, can have devastating consequences for surrounding plant, animal, and human life. An explosion on August 12, 1986, from Lake Nyos in western Cameroon caused a 100-meter-high jet of water and CO₂ gas, coinciding with a 1-m drop in lake level. An approximately 50-m-thick mist of water and CO₂ rolled down into the surrounding valley, at speeds of over 50 km per hour, killing 1,700 people through suffocation (Freeth and Kay, 1987). The buildup of CO₂ occurred in the lower portions of the lake because the confining pressure of the overlying water mass caused CO₂ derived from the underlying volcanic sourceto be dissolved and effectively trapped in the bottom waters. Rainwater displaced some of the bottom waters, leading to reduced confining pressure and explosive gas expulsion. Pipes have been inserted into the bottom of the lakes to allow CO₂ to gradually escape and prevent explosive overturning (Evans et al., 1993).

Volcanogenic gas is also being emitted near Mammoth Mountain, California. After an earthquake swarm in 1989 associated with a moving subterranean magma body, U.S. Forest Service personnel noticed an area where the trees appeared to be dying. In 1990, measurements of gas emis-