This document sets out the recommended method for determining occupational exposure limits for Certain Refined Hydrocarbon Solvent Vapour Mixtures such as Petroleum ether, Rubber solvent and VM&P Naphtha.

The information is based on Appendix H in the ACGIH publication, 2015 Threshold Limit Values and Biological Exposure Indices. While Certain Refined Hydrocarbon Solvent Vapour Mixtures, including Petroleum ether, Rubber solvent and VM&P Naphtha, are not included in the Ontario Table or the ACGIH Table referred to in Regulation 833, an employer has a duty and shall take all measures reasonably necessary in the circumstances to protect workers from exposure to a hazardous biological or chemical agent. The Ministry recommends that OELs for these mixtures be calculated using this method.

Application

The determination of occupational exposure limits OELs for certain refined hydrocarbon solvent vapour mixtures requires the use of the reciprocal calculation procedure (RCP) developed by the American Conference of Governmental Industrial Hygienist (ACGIH). This procedure was first published in Appendix H of the ACGIH publication, 2009 Threshold Limit Values and Biological Exposure Indices.

The RCP is a special use application. The method is intended for calculating OELs for mixtures of certain refined hydrocarbon solvents derived from petroleum consisting of up to 200 components comprising of  aliphatic (alkane), cycloaliphatic (cycloalkanes)  and aromatic hydrocarbons with a carbon number of C5 to C15 and having boiling points in the range of 35° – 329°C. It does not always replace OELs but calculates a guidance OEL (e.g. GGV mixture) based on the composition of a specific complex mixture.

The RCP is only applicable when chemical constituents of the petroleum-based refined hydrocarbon solvent have similar toxicity and the toxicological effects act in an additive manner. The principal toxicological effects of hydrocarbon solvents are acute central nervous system depression and eye and respiratory tract irritation. The RCP is suitable for the determination of OELs for certain refined hydrocarbon solvent mixtures such as Rubber solvent and VM & P Naphtha. GGVs are not appropriate for compounds that do not have either central nervous system impairment or irritation effects.

The RCP can only be applied to vapours of petroleum-based refined hydrocarbon solvents described above, i.e. with specified carbon numbers and boiling point ranges. It does not apply to petroleum derived fuels, lubricating oils or solvent mixtures that already have an OEL. Such components of the mixture should be identified  and sampled individually to ensure exposures are below the OEL. Further, where a mixture is comprised entirely of substances with their own OELs, the formula set out in paragraph 8 of Schedule 1 of Regulation 833 applies.

The use of the RCP should be restricted to applications where the boiling points of the solvents in the mixture are relatively narrow, within a range of less than 45° C (i.e., vapour pressure within approximately one order of magnitude).

Reciprocal calculation procedure (RCP)

The RCP provides the calculation method for the determination of OELs for refined hydrocarbon solvent vapour mixtures. This calculation method known as the reciprocal calculation mixture formula is based on:

  • The mass composition of the mixture
  • Hydrocarbon group guidance values GGVs
  • Substance-specific OELs, where applicable

The hydrocarbon GGVs are categorized based on similar chemical and toxicological properties. The GGVs to be used for the calculation of the OEL of the hydrocarbon mixture are given in Table 1.

Using the reciprocal calculation mixture formula given below, the calculated GGV mixture becomes the 8-hour TWA-OEL for the refined hydrocarbon mixture.

The reciprocal calculation mixture formula

GGV mixture = 1 / (Fa / GGVa + ........ + Fn / GGVn)

Where,

GGV mixture = the calculated 8-hour TWA-OEL for the mixture based on the GGVs given in Table 1

F a = the liquid mass fraction of group (or component) “a” in the hydrocarbon mixture (value between 0 and 1)

F n = the liquid mass fraction of the nth group (or component) in the hydrocarbon mixture (value between 0 and 1)

GGV a = the guidance value (or OEL) for group (or component) “a”

GGV n = the guidance value (or OEL) for the nth group (or component)

Note: The Group Guidance Values for the respective hydrocarbon groups are given in Table 1; the OELs for specific components are prescribed under Regulation 833.

Rounding Rule: GGV mixture should follow recommended rounding techniques. For calculated values <100 mg/m³, round to the nearest 25. For calculated values between 100 and 600 mg/m³, round to the nearest 50. For calculated values > 600 mg/m³, round to the nearest 200 mg/m³.

Limitations

The RCP does not apply to solvents that contain agents such as benzene or n-hexane, or methylnaphthalene. These solvents have individual exposure limits that are significantly less than the GGV to which they would belong and have unique toxicological effect.

The RCP applies only to vapours and does not apply to mists or aerosols.

The calculation does not apply to benzene. It does not apply to mixtures containing olefins (alkenes) or other unsaturated compounds or carcinogenic polycyclic aromatic hydrocarbons (PAHs).

Fa and Fn of the reciprocal calculation mixture formula requires that the mixture be characterized at least to the detail of mass percent of the groups given in Table 1.

Table 1: Refined Hydrocarbon Groupings and GGVs*
Hydrocarbon GroupGroup Guidance Value (GGV)
C5 – C6 Alkanes1500 mg/m³
C7 – C8 Alkanes1500 mg/m³
C5 – C6 Cycloalkanes1500 mg/m³
C7– C8 Cycloalkanes1500 mg/m³
C7 – C8 Aromatics200 mg/m³
C9 – C15 Alkanes1200 mg/m³
C9 – C15 Cycloalkanes1200 mg/m³
C9 – C15 Aromatics100 mg/m³

*The GGVs listed in Table 1 are reproduced from Column B of the ACGIH Table 1 Group Guidance Values found in Appendix H of the ACGIH publication, 2015 Threshold Limit Values and Biological Exposure Indices.

Due to the inherent complexity of the RCP and derivation of group guidance values for mixtures, guidance from an occupational hygiene professional should be sought during application of this method.

Examples

The following two examples outline sample calculations for selected refined hydrocarbon solvents.

Example 1

A solvent has the following mass composition and assigned GGVs applying Table 1.

Hydrocarbon GroupMass FractionGroup Guidance Value
C7 – C8 Alkanes45%1500 mg/m³
C9 – C10 Alkanes / cycloalkanes40%1200 mg/m³
C7 – C8 Aromatics9%200 mg/m³
Toluene6%75 mg/m³ (TWA-OEL)
Benzene< 1%Not applicable*

*The RCP does not apply to benzene as the toxicity of benzene is significantly higher and has an exposure limit much lower than the GGVs of the mixtures at large.

Based on the above compositional information, the reciprocal calculation procedure is applied to determine the GGVmixture, that is, the calculated 8-hour TWA - OEL for the mixture.

GGVmixture = 1 / (0.45 / 1500 + 0.40 / 1200 + 0.09 / 200 + 0.06 / 75)
= 1 / 0.001883
= 530 mg/m3 or rounded to 550 mg/m3

Therefore, the GGVmixture for the above solvent is 550 mg/m³.

See rounding rule, reciprocal calculation mixture formula

Example 2

Another solvent has the following mass compositions and assigned GGVs applying Table 1.

Hydrocarbon GroupMass FractionGroup Guidance Value
C5 – C8 Alkanes30%1500 mg/m³
C9 – C15 Alkanes/cycloalkanes50%1200 mg/m³
C7 – C8 Aromatics10%200 mg/m³
C9– C15 Aromatics5%100 mg/m³
Toluene4%75 mg/m³ (TWA-OEL)
Benzene< 1%Not applicable*

*The RCP does not apply to benzene as the toxicity of benzene is significantly higher and has an exposure limit much lower than the GGVs of the mixtures at large.

GGVmixture = 1 / (0.3/1500 + 0.5 / 1200 + 0.1 / 200 + 0.05 / 100 + 0.04 / 75)
= 1 / 0.00215
= 465 mg/m3 or rounded to 450 mg/m3

Therefore, the GGVmixture for the solvent in Example 2 is 450 mg/m³.