How will an employer determine a “safe for entry” level for contaminants under the provisions of paragraph (c)(5)?
OSHA is willing to accept as the minimal “safe for entry” level, that which is 50% of the flammable or toxic substance that would constitute a hazardous atmosphere. The two examples footnoted on page 4488 of the preamble to the final rule are: (1) The LFL for methane is a concentration of 5 percent by volume. Ten percent of this value is 0.5 percent, a concentration which would be considered hazardous by definition. Under the guideline the measured concentration of methane cannot exceed 0.25 percent after ventilation in order for the procedures specified in paragraph (c)(5)(ii) of the final rule to be acceptable. (2) The 8-hour time weighted average PEL for chlorine, under Table Z-1, is 1.0 parts per million. This concentration of chlorine would be considered hazardous by the definition of “hazardous atmosphere”. Under the guideline, the measured concentration of chlorine cannot exceed 0.5 parts per million after ventilation in order for the procedures specified in paragraph (c)(5)(ii)
