1. Conversion from ppm to mg/m3:
C (mg/m3) = C (ppm) x MW / 24.45
- C (mg/m3) = exposure concentration in mg/m3
- C (ppm) = exposure concentration in ppm
- MW = molecular weight of the chemical of interest
2. Calculation of duration-adjusted exposure levels in mg/m3 for experimental animals.
NOAEL [ADJ] (mg/m3) = E (mg/m3) x D / 24 x W / 7
- NOAEL [ADJ]= the NOAEL adjusted for the duration of an experimental regimen
- E = experimental exposure level (NOAEL)
- D = number of hours of exposure per day
- W = number of days of exposure per week
For example: the NOAEL[ADJ] for an experimental NOAEL of 30 mg/m3 for 6 hours per day and 5 days a week is 30 (mg/m3) x 6/24 x 5/7 = 5.4 mg/m3.
3. Calculation of human equivalent concentration
NOAEL[HEC] (mg/m3) = NOAEL[ADJ] (mg/m3) x DAFr
- NOAEL[HEC] = human equivalent NOAEL
- NOAEL[ADJ]= the NOAEL adjusted for the duration of an experimental regimen
- DAFr = dosimetric adjustment factor: either the regional deposited dose ratio (RDDRr) for particles or the regional gas dose ratio (RGDRr) for gases;
-r = the affected respiratory tract region: e.g., thoracic (TH), tracheobronchial (TB), pulmonary (PU), extrathoracic (ET), or total (T).
Note: RDDRr can be calculated by using the RDDR program that comes with the RfC guidelines (U.S. EPA, 1994). RGDRr can be calculated based on the instructions in the RfC guidelines (U.S. EPA, 1994).
U.S. EPA 1994. Methods for derivation of inhalation reference concentrations and application of inhalation dosimetry. EPA/600/8-90/066F, Oct. 1994.
4. Dosimetric adjustment for human occupational study data
NOAEL[HEC] = NOAEL (mg/m3) x (VEho/VEh) x W / 7