Print Print This Page

Determination of Occupational Exposure Limits for Certain Refined Hydrocarbon Solvent Vapour Mixtures

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, 2011 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 ACGIH (American Conference of Governmental Industrial Hygienists). 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 containing saturated aliphatic, cycloaliphatic and aromatic hydrocarbons consisting of 5 to 15 carbon atoms and having boiling points in the range of approximately 35° – 320° C. Mixtures of these hydrocarbon solvents may consist of up to hundreds of components comprising of aliphatic (alkane), cycloaliphatic (cycloalkanes), and aromatic hydrocarbons.

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.

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. It also does not apply to hydrocarbons with toxicity significantly higher than the mixture at large such as benzene, n-hexane, and methylnaphthalene. Such components of the mixture should be measured and considered individually. Further, where a mixture is comprised entirely of substances with their own OELs, the formula set out in paragraph 7 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 subscript mixture equals one divided by the sum of the results of F subscript a divided by GGV subscript a plus, up to and including, the results of F subscript n divided by GGV subscript n.

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 effects.

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

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

Fa and F n 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 Group Group Guidance Value (GGV)
(mg/m³)
C5 – C6 Alkanes 1500
C7 – C8 Alkanes 1500
C5 – C6 Cycloalkanes 1500
C7– C8 Cycloalkanes 1500
C7 – C8 Aromatics 200
C9 – C15 Alkanes 1200
C9 – C15 Cycloalkanes 1200
C9 – C15 Aromatics 100

[*] 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, 2009 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 Group Mass Fraction (%) Group Guidance Value (mg/m³)
C7 – C8 Alkanes 45 1500
C9 – C10 Alkanes/cycloalkanes 40 1200
C7 – C8 Aromatics 9 200
Toluene 6 75 (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.

GGV subscript mixture equals one divided by the sum of the results of zero decimal four five divided by one thousand five hundred plus zero decimal four zero divided by one thousand two hundred plus zero decimal zero nine divided by two hundred plus zero decimal zero six divided by seventy-five.  Equals one divided by zero decimal zero zero one eight eight three. Equals five hundred and thirty milligrams per metres cubed or rounded superscript asterisk to five hundred and fifty milligrams per metres cubed.

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 Group Mass Fraction (%) Group Guidance Value (mg/m³)
C5 – C8 Alkanes 30 1500
C9 – C15 Alkanes/cycloalkanes 50 1200
C7 – C8 Aromatics 10 200
C9– C15 Aromatics 5 100
Toluene 4 75 (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.

GGV subscript mixture equals one divided by the sum of the results of zero decimal three divided by one thousand five hundred plus zero decimal five divided by one thousand two hundred plus zero decimal one divided by two hundred plus zero decimal zero five divided by one hundred plus zero decimal zero four divided by seventy-five. Equals one divided by zero decimal zero zero two one five. Equals four hundred and sixty-five milligrams per metres cubed or rounded superscript asterisk to four hundred and fifty milligrams per metres cubed.

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