As companies innovate and redevelop products, a change in the manufacturing process or an introduction of a new process are becoming common occurrences. These changes can involve the introduction of complex chemical mixtures. The use of complex chemical mixtures poses many safety challenges. Frequent questions include:
- Can workers be overexposed by inhalation?
- Is the mixture a flammability hazard?
- How do I measure exposure?
- Are engineering controls adequate?
For the best results, company should engage a safety professional to evaluate if a chemical mixture is safe to use.
The client company, a small innovator in the food industry, has implemented new processes. When the executives suspected that their new production process was causing a possible overexposure issue, they looked for a niche expert in occupational hygiene. As a small producer in a competitive market, they had a small budget to spend and a tall order of expectations to meet.
The goal of the client company is to be a quality local alternative to imported items in their category. As a result, they were looking for a solution that would guarantee that their product and process remain safe for consumers and workers. As a part of the overall corporate strategy, ensuring occupational safety was more than just a legal precaution – it helped gain a competitive edge. Importers could not guarantee worker safety in their production facilities and did not provide insight into product safety. Overall, the company was aiming for a certified niche expert who could understand and ensure the safety of their operations. The solution had to be (1) cost-effective, yet (2) timely and (3) professional. The executives chose THEM as a reliable consultant with proven expertise and a history of success since 1979.
This project required THEM to assess the potential occupational exposure to a paint primer. A paint primer is a complex mixture of chemicals. Current assessment techniques and laws for chemicals tend to focus on potential exposure to individual chemicals, mostly considering only a single source. This practice is concerning, because, as a result, manufacturers rarely examine potential risks of these chemicals in combination. Although, scientists and regulators develop and apply methodologies for assessing the combination effects of chemicals, so far there is no systematic, consistent, comprehensive and integrated approach across different pieces of legislation.
Harris Environmental Management Inc. (THEM) are experts in advance sampling strategies and other exposure determinants to assess what is significant or important with respect to a chemical mixture in order facilitate professional judgement on assessing the safety of the mixture.
Evaluating Occupational Exposure to Paint Primer
Finding the Focus: Currently, in the production processes, the variety of chemicals and their possible combinations is so great that it neither realistic nor useful to test every combination. Where do we begin in assessing the complex mixture of paint primer? The assessment begins with a qualitative step to prioritize exposures to minimize the need for the sampling.
A quick look at SDS: Looking at the manufacturer’s safety data sheet (SDS), consultants determined that this primer paint contains three main volatile solvent ingredients in the coating mixture: (83-93%) 1-methoxypropyl-2-acetate, (<4%) 2-methoxypropyl-1-acetate and (2-7%) ethanol. The remaining ingredients are proprietary polymers with no vapour pressure according to the SDS. They do not contribute to the volatile organic compounds and can be left out of the vapour pressure analysis of the mixture.
Further analysis/modelling: Using the AIHA fundamental heuristics of Rule-of-Ten (ROT) and Vapour Hazard Ratio (VHR) the consultant can assess complex solvent mixtures.
The Rule-of-Ten correlates the fraction of saturated vapour pressure with the level of control. The fraction of saturation is an approximation of the 95th percentile and should not be considered an absolute exposure level but rather be used as a tool to provide the correct order of magnitude of exposure.
Vapour Hazard Ratio (VHR) is an indication of the potential of the agent to exceed acceptable airborne levels. The American Industrial Hygiene Association have linked VHR to the required level of ventilation control necessary to maintain the 95th percentile exposure below the agent’s occupational exposure limit to assure worker exposures are acceptable. The significance of exposure is related to both tendency for a material to vapourize and levels of concentration in the air that can cause harmful effects.
Table 1. Vapour Hazard Ratio Linked to Ventilation
Table 1 associates ranges in VHR with the required level of ventilation control necessary to assure worker exposures are acceptable.
Finding the controlling component: Depending on partial pressures and concentration, the controlling compound with the highest adjusted VHR can be determined in a chemical mixture. If occupational exposure air monitoring shows that the controlling ingredient is below the occupational exposure limit, then all chemical ingredients will be in control. If such an ingredient is identified, it can drastically limit the need for multiple chemical air sampling and the site can focus on the only on the controlling compound.
Using Raoult’s Law for ideal solutions the consultant is able to calculate partial pressures for each chemical ingredient in the solvent mixture and then determine the adjusted vapour hazard ratios. With this method, consultants calculated that of the three volatile solvent ingredients present in the mixture, 1-methoxypropyl-2-acetate, also commonly known as propylene glycol methyl ether acetate, has the highest adjusted vapour hazard ratio of 0.062 and was the controlling component for occupational exposure monitoring. This VHR corresponds to a Vapour Hazard Ratio Scale of 2 from Table 1, which indicates that good general ventilation between 6 – 12 air changes per hour is the required level of ventilation to control occupational exposure to this paint primer.
Determining the necessary ventilation control measures: If we use the corrected vapour pressures for the ingredients to calculate the saturation vapour concentrations for each ingredient and multiply by the fraction of saturation value for each level of control from the Rule of Ten, we get the following relative risk determination based on ventilation control as per Table 2 below.
Table 2. Saturated Vapour Pressure Calculation & Relative Risk Determination By Ventilation Control Parameters (Rule-of-10)
These calculations showed that confined space ventilation conditions (very limited) are required before the three chemical ingredients exceed the occupational exposure limits. Even under poor ventilation conditions (< 6 air changes per hour), occupational exposures to this solvent mixture would be acceptable.
1. THEM were able to demonstrate to the client that 1-methoxypropyl-2-acetate is the controlling solvent in this mixture. If air sampling is to be conducted, then this is the chemical ingredient that should be measured thus saving the client money on analytical costs for the other two chemical ingredients.
2. THEM showed that general good ventilation of 6 – 12 air changes per hour would control occupational exposures below the occupational exposure limit.
3. THEM found that concentrations in a confined space would not exceed the ethanol LEL of 3.3% (33,000 ppm) or of 1-methoxypropyl-2-acetate) LEL of 1.3% (13,000 ppm) (78°C) and the paint primer is not an explosion hazard.
 S. Jahn, W. Bullock, J. Ignacio – A Strategy for Assessing and Managing Occupational Exposures – 4th Edition, American Industrial Hygiene Association, Chap. 26, p. 335-348, 2015