Calculating the total reduced sulphur threshold

This page provides information on how to calculate the total reduced sulfur (TRS) threshold when you are reporting to the National Pollutant Release Inventory (NPRI).


TRS refers to a gaseous mixture of compounds containing one or more sulphur atoms in their reduced state. For the purposes of reporting to the NPRI, this grouping is restricted to the substances listed in Table 1 below. It is important to remember that three of these TRS compounds – hydrogen sulphide (H2S), carbon disulphide (CS) and carbonyl sulphide (COS) – are also listed individually under Part 1A. If any one of these three substances meets the 10 tonne reporting threshold, that substance must also be reported individually.

Table 1. Total reduced sulphur substances
Substance name Chemical formula Chemical Abstract Service Registry Number Hydrogen sulphide equivalence factor
Hydrogen sulphide H2S 7783-06-4 1.00
Carbon disulphide CS2 75-15-0 0.895
Carbonyl sulphide COS 463-58-1 0.567
Dimethyl sulphide C2H6S 75-18-3 0.548
Methyl mercaptan CH4S 74-93-1 0.708
Dimethyl disulphide C2H6S2 624-92-0 0.724

Determining TRS threshold and emission quantities

TRS is a Part 1A substance and subject to the Part 1A reporting thresholds.  The manufactured, processed or otherwise used (MPO) threshold for this substance is 10 tonnes for a given calendar year. When calculating TRS quantities, for threshold and estimate purposes, TRS must be expressed in terms of H2S. TRS quantities can be determined using several methods, including summing H2S equivalencies, emissions monitoring, or source testing. This document explains how to determine the reporting threshold for TRS using H2S equivalence factors.

Equivalence factor method

To use the equivalence factor method, each TRS compound is converted into the H2S equivalent.  The equivalent amounts are then summed and compared to the 10 tonnes MPO threshold.  If the threshold is met or exceeded, a TRS substance report is required.  The H2S equivalence factors are listed in Table 1 above.


Table 2 illustrates how you calculate the MPO reporting threshold for TRS using the equivalence factor method. To calculate the amounts expressed as H2S, add the quantities of the substances multiplied by the H2S applicable equivalence factors:

TRS = H2S + (CS2 × 0.895) + (COS × 0.567) + (C2H6S × 0.548) + (CH4S × 0.708) + (C2H6S2× 0.724)

Table 2. Total reduced sulphur reporting threshold example calculation
Substance name Chemical formula Quantity MPO'd (tonnes) H2S equivalent factor Equivalent
H2S MPO'd (tonnes H2S)
Hydrogen sulphide H2S 3 1.000 3.000
Carbon disulphide CS2 2 0.895 1.790
Carbonyl sulphide COS 22 0.567 12.481
Dimethyl sulphide C2H6S 4 0.548 2.194
Methyl mercaptan CH4S 2 0.708 1.417
Dimethyl disulphide C2H6S2 6 0.724 4.341
Total reduced sulphur (expressed as H2S) 25.223

In this example, a TRS substance report is required (assuming the facility also meets the 20,000 hour employee threshold or an activity to which the employee threshold does not apply takes place at the facility), because the total amount of TRS (25.223 tonnes) exceeded 10 tonnes for the calendar year. In addition, COS must be reported separately, since the 10 tonne threshold was met for that substance (12.481 tonnes) and COS is listed separately as a Part 1A substance. 

You will report air release amounts of TRS even if the values are zero. However, you are not required to report other release, disposal or recycling amounts even if the values are zero. For COS, you will report the release, disposal and recycling amounts, even if the values are zero.  

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