Averaging periods
 17. (1) If a provision of this Part refers to an approved dispersion model being used in connection with a standard that applies to a specified averaging period, the following rules apply for the purposes of this Part: 1. If an approved dispersion model was designed to be used for the specified averaging period, it shall be used as it was designed for that averaging period. 2. If an approved dispersion model was not designed to be used for the specified averaging period but was designed to be used for an averaging period shorter than the specified averaging period, the model may be used as it was designed for the shorter averaging period if the result produced by the model is adjusted in accordance with subsection (2). 3. If the specified averaging period is less than one hour and an approved dispersion model was designed to be used for a one hour period, the model may be used as it was designed for a one hour period if the result produced by the model is adjusted in accordance with subsection (2). 4. If the use of an approved dispersion model is not authorized or required by paragraph 1, 2 or 3, the model shall not be used.  O. Reg. 419/05, s. 17 (1). (2) If a provision of this Part refers to an approved dispersion model being used in connection with a standard that applies to a specified averaging period, (a) the result produced by the approved dispersion model shall be multiplied by the amount determined in accordance with subsection (3), if the model is used under paragraph 2 of subsection (1); and (b) the result produced by the approved dispersion model shall be divided by the amount determined in accordance with subsection (3), if the model is used under paragraph 3 of subsection (1).  O. Reg. 419/05, s. 17 (2). (3) The amount referred to in clauses (2) (a) and (b) is the amount determined in  accordance with the following formula: (t0 ÷ t1)n where, t0 = the shorter of, i. the averaging period that the approved dispersion model was designed to be used for, expressed in hours, and ii. the specified averaging period, expressed in hours, t1 = the longer of, i. the averaging period that the approved dispersion model was designed to be used for, expressed in hours, and ii. the specified averaging period, expressed in hours, n = 0.28 or, if the Director is satisfied that another number would result in an adjustment that produces a more accurate prediction of the concentration of the relevant contaminant, the other number. O. Reg. 419/05, s. 17 (3). (4) If an approved dispersion model is used with respect to a person and contaminant to whom section 20 applies and Schedule 3 sets out more than one standard for the contaminant, using different averaging periods, the model shall be used with respect to each averaging period.  O. Reg. 516/07, s. 13. (5) This section does not apply if, (a) the approved dispersion model that is used is the ASHRAE method of calculation and the model is being used in connection with a standard set out in Schedule 3 that applies to a 10 minute averaging period; or (b) the approved dispersion model that is used is the method of calculation required by the Appendix to Regulation 346.  O. Reg. 516/07, s. 13.