Avoid mishaps with oil mist control best practices

Metalworking applications that require coolants or lubricants produce aerosolized particulates composed of tiny oil droplets. While they are often too small to see with the naked eye, they can jeopardize worker safety and productivity.

Metalworking fluids generally can be categorized as one of four types based on their different properties and recommended applications.

1. Straight oil. This can be mineral — that is, petroleum — or vegetable oil and mainly lubricates rather than cools.

2. Soluble oil. Composed of 30% to 85% refined petroleum-based oil, along with emulsifiers and other additives, soluble oil lubricates and cools but may result in more smoke and residue than straight oil produces.

3. Semisynthetic. This contains 5% to 30% petroleum-based oil, along with a complex emulsifier package. Semisynthetic provides good lubrication and heat reduction and is cleaner than soluble oil.

4. Synthetic. This is composed of detergentlike compounds rather than petroleum. Synthetic is usually the cleanest option and effectively reduces heat.

Oil mist characteristics vary according to the type of oil and the machining process. Petroleum-based oil tends to produce more solid particulates, as well as aerosolized oil particles, than other metalworking fluids, creating a dirtier oil mist.

Low-speed cutting and grinding produce relatively large, mechanically generated particles that are easy to collect in filters and consequently of less concern. The particles are generally about 10 µm (0.0004″) in diameter.

On the other hand, high-speed processes likely cause a lot of heat, creating thermally generated droplets. It vaporizes the liquid, which results in aerosols that tend to be less than 1 µm (0.00004″) in size. This oil mist usually looks more like smoke and is harder to filter than mechanically generated droplets.

As the speed of production continues to increase and cycle times decrease, more heat is being created. That means there are more processes in which mists are generated thermally, resulting in smaller particles than before. This change presents a filtration challenge.

The smaller the particle, the more difficult it is to clean the air. More efficient filters therefore are needed to mitigate the problem.

Slip and Slide

The most obvious, immediate safety issue occurs when oil mist causes workers to slip and fall. It often hangs in the air before forming a layer of grease on floors and other surfaces. Heavy oil mist droplets coalesce as they cool and fall onto horizontal surfaces.

Oil mist also poses health problems when inhaled. This risk varies with the type of oil used, the additives in the metalworking fluid and the way the mist is generated. Some mists contain chemicals that irritate lungs, as well as skin, eyes, the nose and the throat.

As previously noted, widely performed high-speed thermal processes tend to produce small droplets that can be inhaled deep into lungs. All oil mists can produce respiratory irritation, and prolonged exposure has been linked to asthma, chronic bronchitis, impaired lung function and pulmonary fibrosis of the lungs. Some metalworking fluids cause skin irritation or contact dermatitis when they settle on skin.

Finally, there is evidence that some metalworking fluids increase the rates of certain cancers, though this has not been proven definitively.

Fortunately, appropriate collection and filtration reduce potential health risks and increase worker satisfaction and productivity.

Machining processes often can be contained under hoods to reduce worker exposure to hazardous oil mist. But even if humans are not exposed, it is important to control oil mist in enclosures to protect equipment. As oil mist builds up in an enclosure, oil mist can make its way into electronic components, such as programmable logic controllers, leading to premature failure. Potential losses can be minimized by keeping moisture levels low in enclosures.

An operator must be able to reach into an enclosure to load and unload parts. If it is not evacuated correctly, a worker may come into contact with a misty area. Parts also can drip oil as they travel on a bin or tray, creating an additional
hazard. Furthermore, many machine tools have optics to measure surface quality. Any mist in an enclosure can affect them.

Limiting Exposure

To reduce the risk of adverse health effects, the Occupational Safety and Health Administration sets permissible exposure limits for workers at facilities where contaminants are a concern. These limits set the amount of a substance that a worker can be exposed to over the course of an eight-hour shift.

For mineral-based oil mists — paraffin oil mists, heavy mineral oil mists, white mineral oil mists and petroleum-based cutting oils — the permissible exposure limit for general industry, including automotive manufacturing, is set at 5 mg/m³ (0.005 ppm) as an eight-hour time-weighted average. All other metalworking fluids fall under general OSHA requirements for “particulates not otherwise categorized.” The permissible exposure limit for these oil mists is 15 mg/m³ (0.015 ppm).

OSHA establishes the only regulatory limits for oil mists. However, the National Institute for Occupational Safety and Health and the American Conference of Governmental Industrial Hygienists have established stricter recommendations for oil mist limits, especially for metalworking fluids that are not mineral-based.

NIOSH’s recommended exposure limit for all metalworking fluid aerosols is just 0.4 mg/m³ (0.0004 ppm) for thoracic particulate mass as a time-weighted average concentration for up to 10 hours per day during a 40-hour workweek.

ACGIH, which is made up of a group of industry professionals, maintains its own list of air quality standards. These are typically more stringent than OSHA’s standards, and OSHA often changes its standards to match ACGIH’s guidelines.

ACGIH’s threshold limit value for mineral oils is 5 mg/m³ for an eight-hour time-weighted average and 10 mg/m³ (0.01 ppm) for a 15-minute short-term exposure limit. Other metalworking fluids do not have a specific threshold limit value, but keeping exposures as low as possible is recommended.

As more manufacturers switch from straight oil to synthetic, it’s important to note that plenty remains unknown about the exposure risks related to synthetic oil. This is also true of the detergents, emulsifiers and other additives used to enhance performance.