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Smoke is one of those simple special effects that never fails. Puccini's magnificent Te Deum gains in dark grandeur when the sanctuary of Sant'Andrea della Valle is filled with billowing clouds of incense. And a whole host of basso bad guys seem just a little more sinister in hazy surroundings.

"Smoke makes the air a palpable presence," says designer John Conklin. "You can see the light moving ... it's a way of making the beams of light register."

It looks good, but is it safe to breathe? Despite a number of studies, it's been difficult to determine the effects of long-term exposure to various types of fog. "Management is beginning to understand that there may be a problem, and consider that they might have a moral responsibility not to endanger singers and crew," says Conklin, who is also director of productions at Glimmerglass Opera and New York City Opera.

Most often, creating fog effects involves heating a commercial fog fluid (such as Rosco Fog Fluid) to a vapor state. The resulting vapor is then forced through a delivery nozzle and into the local environment, where it forms an aerosol fog.

Though the exact composition of many fog fluids is a "trade secret," studies have identified them to be a mixture of multiple glycols and water. Glycols as a group are used as solvents for the pharmaceutical, food additives, cosmetics, inks, and lacquer industries, and in heat exchangers, antifreeze formulations, hydraulic fluids, and chemical intermediates. They are also used as industrial solvents for nitrocellulose and cellulose acetate.

Some groups have questioned the effects of long-term exposure to glycols and have proposed limitations on exposure to their vapors. Though a dizzying array of interest groups have conducted studies and set guidelines, there is still no consensus on safe usage. The Occupational Health and Safety Administration (OSHA) has set safe inhalation exposure concentrations for glycerin, but not for the other five glycols used in fog production. Actor's Equity Association and the Worker's Compensation Board of BC have set their own guidelines for materials that may be used in achieving fog effects.

"Keeping the stage safe is of ultimate importance to all production personnel," says Debra Harrison, director of productions at Vancouver Opera. "While it's very easy to assess the possible hazards of almost all theatrical effects, smoke continues to be an endless unknown. Though there has been a drastic increase in the amount of research being done regarding the use of the various types of smoke effects, there are still precious few confirmed safety standards for its use."

While some studies found no harmful effects from glycol-based fog, others have shown that large amounts of glycol vapors may have a drying effect on mucous membranes. Another safety concern has to do with the toxicity of by-products of the fogging process: Acrolein, acetaldehyde, and formaldehyde are potential decomposition products of glycols, and all are established to be strongly irritating to the eyes and respiratory tract. Some studies have found low levels of these chemicals in air samples from productions using fog, especially when fluids were heated at higher temperatures.

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