How to Handle Methylene Chloride
A solvent not to ignore
This article provides vital safety information on one of the most widely used laboratory solvents. It is probably best known for its common use outside laboratories (i.e. a chemical stripper or paint and varnish remover). If you have ever tried refinishing old treasured furniture or removing paint to restore wood trim, you have most likely used Strypeeze™, the orange stuff, or a similar paint/varnish stripper from the local hardware store. The chemical stripper is predominately methylene chloride.
Also known as dichloromethane, methylene chloride is a potentially dangerous solvent responsible for at least 19 fatalities since 2006, three of which occurred in 2017, the last year published data is available. The CDC reported that OSHA identified 10 deaths related to methylene chloride stripping agents and another three were investigated by the Michigan FACE (Fatality Assessment and Control Evaluation) program during 2000 to 2011. Granted, these types of fatalities are not likely to occur in our laboratories, but if you stop to consider that the average amount used in each case was only six fluid ounces (177ml), and that exposures as short as one hour was all that was necessary, the fatalities demonstrate vividly the potential dangers of working with methylene chloride.
Methylene chloride and its associated hazards are serious enough for OSHA to have produced a specific standard covering its use in the workplace—29CFR1910.1052. The standard sets contaminant action levels, permissible exposure limits, and requirements for compliance—details that we will distill and present for you below. But first we will explore a little more about the dangerous solvent we are using.
What you need to know: chemical/physical properties, symptoms, and effects
In typical laboratory settings, the most common use for methylene chloride is as a solvent, especially as an extraction liquid for gas chromatography. Other uses include decaffeinating coffee and teas, metal cleaning and degreasing, pharmaceutical and adhesive manufacturing, polyurethane foam and polycarbonate resin production, and chemical stripping, among many others.
It is a clear, colorless liquid with a moderately sweet aroma. Methylene chloride is highly volatile with a low boiling point (104°F) and vapor pressure (350mm Hg). Combined with its heavier-than-air molecular weight (85), methylene chloride is a serious inhalation hazard.
The primary exposure route for methylene chloride is via inhalation, although absorption through the skin is also a concern. Acute inhalation produces central nervous system depression and can lead to narcosis at very high concentrations, eventually causing respiratory failure and death. Since methylene chloride is metabolized to formaldehyde and carbon monoxide, chronic exposures can produce carbon monoxide-type symptoms of headache, nausea, vomiting, confusion, and dizziness. Skin contact can result in irritation and chemical burns. In addition, OSHA considers methylene chloride a potential occupational carcinogen.
Know your exposure risk
The OSHA standard covers all occupational exposures to methylene chloride in general industry (separate standards cover shipyards and the construction industry). If this solvent is used in your workplace, an exposure assessment and hazard evaluation is required for those employees handling and using the material.
The OSHA action level (AL) is 12.5 ppm (parts per million methylene chloride in air) and if this concentration is reached or exceeded, compliance activities such as monitoring and medical surveillance are triggered. The permissible exposure limit (PEL) is 25 ppm, at which point employers must use engineering and work practice controls to limit employee exposures. Both the AL and the PEL are based on eight-hour time weighted averages (TWA) or, in other words, an average exposure for a full work shift.
Respiratory protection is an alternative means of protection, only as an interim measure while engineering controls are put in place or if they are insufficient or unavailable.
There is also a short-term exposure limit (STEL) of 125 ppm based on a 15-minute TWA. This level should never be exceeded. Most importantly, the immediately dangerous to life and health limit is 2300 ppm.
Related Article: Stripper, Anyone?
Assessments are conducted by measuring the air concentrations near the worker’s breathing zone for a representative number of employees for each process or task where the chemical is used. An initial assessment and monitoring are required unless the employer has objective data that demonstrate the highest potential exposure (worst case scenario) will not exceed the AL and STEL or potential exposures would occur less than 30 days per year.
Understand compliance requirements
Once the initial assessments are completed, the data are evaluated. If the AL and/or STEL are exceeded, then periodic monitoring is required following table 1 below.
Table 1: Monitoring Requirements | |
Exposure Scenario | Required Monitoring Activity |
| Below the action level (12.5 ppm) and at or below the STEL (125 ppm) | No eight-hour TWA or STEL monitoring required. |
| Below the action level (12.5 ppm) and above the STEL (125 ppm) | No eight-hour TWA monitoring required; monitor STEL exposures every three months. |
| At or above the action level (12.5 ppm), at or below the PEL (25 ppm TWA), and at or below the STEL (125 ppm) | Monitor eight-hour TWA exposures every six months. |
| At or above the action level (12.5 ppm), at or below the PEL (25 ppm TWA), and above the STEL (125 ppm) | Monitor eight-hour TWA exposures every six months and monitor STEL exposures every three months. |
| Above the PEL (25 ppm TWA), and at or below the STEL (125 ppm) | Monitor eight-hour exposures every three months. |
| Above the PEL (25 ppm TWA) and above the STEL (125 ppm) | Monitor eight-hour TWA exposures and STEL exposures every three months. |
Based on the initial assessments, establish regulated areas and clearly mark all spaces where the PEL and STEL levels are exceeded. Include any area where the limits are expected to go above the PEL or STEL. Minimize the number of employees authorized to enter these areas.
Hazard communication must inform all affected employees on the dangers of working with methylene chloride, including the health effects, symptoms of exposure, and safety requirements. Ensure appropriate training is given prior to beginning work and periodically after, and that employees demonstrate competency.
Finally, employers must implement a medical surveillance program and include every employee covered by the OSHA standard. Medical surveillance should be provided at no cost to the employee and should include:
- An initial physical exam and medical history
- Periodic exams based on the employee’s age
- Emergency exams following any incident and at the
end of employment or upon re-assignment.