For many years, sulfur dioxide levels have been determined through the use of lead peroxide candles. These devices, known as candles because they are a mixture of lead peroxide paste spread on a porcelain cylinder about the size and shape of a candle, are normally exposed for periods of 1 mon. Sulfur gases in the air react with lead peroxide to form lead sulfate. Environmental engineers analyze the sulfate according to standard laboratory procedures to indicate the atmospheric levels of sulfur gases during the period of exposure.
A modification of this technique that simplifies the laboratory procedure is to use a fiber filter cemented to the inside of a plastic petri dish (a flat-bottom dish with shallow walls used for biological cultures). The filter is satu
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