29 CFR §1915.1000

1. (a)
   1. (1)Substances with limits preceded by “C”—Ceiling values. An employee's exposure to any substance in Table Z—Shipyards, the exposure limit of which is preceded by a “C,” shall at no time exceed the exposure limit given for that substance. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time over a working day.
   2. (2)Other Substances—8-hour Time Weighted Averages. An employee's exposure to any substance in Table Z—Shipyards, the exposure limit of which is not preceded by a “C,” shall not exceed the 8-hour Time Weighted Average given for that substance in any 8-hour work shift of a 40-hour work week.
2. (b)-(c) [Reserved]
3. (d)Computation formula. The computation formula which shall apply to employee exposure to more than one substance for which 8-hour time weighted averages are listed in subpart Z of 29 CFR part 1915 in order to determine whether an employee is exposed over the regulatory limit is as follows:
   1. (1)
      1. (i)The cumulative exposure for an 8-hour work shift shall be computed as follows:
      2. (ii)To illustrate the formula prescribed in paragraph (d)(1)(i) of this section, assume that Substance A has an 8-hour time weighted average limit of 100 ppm noted in Table Z—Shipyards. Assume that an employee is subject to the following exposure:
   2. (2)
      1. (i)in case of a mixture of air contaminants an employer shall compute the equivalent exposure as follows:
      2. (ii)To illustrate the formula prescribed in paragraph (d)(2)(i) of this section, consider the following exposures: