А что, контекста совсем нет? О чём в целом текст? Дайте хотя бы предложение с этим PERC. Может, это просто сокращение (от percentage, например). Tetrachloroethylene (PCE, perc) in Air PCE: ▪ is a nonflammable, colorless liquid or gas ▪ evaporates easily into air ▪ smells sweet at high concentrations; at lower levels, there is no odor http://www.health.state.mn.us/divs/eh/hazardous/topics/vapintruspce.pdf Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene ("perc" or "PERC"), and many other names, is a chlorocarbon with the formula Cl2C=CCl2.
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