Household appliances are not designed to withstand the hazardous materials utilized in a lab nor the processes in which they are utilized. Household microwaves should not be used in a laboratory.

Microwave ovens are used for heating and defrosting in laboratories. However, improper use of a microwave can pose a number of hazards including:

  • Ignition of flammable vapors
  • Electrical shock from ungrounded or faulty units
  • Ignition of materials being heated
  • Pressure build-up in sealed containers
  • Integrity of containers holding materials
  • Sudden boiling of liquid in an open container following removal

Steps to Working Safely with Microwaves

  • Never attempt to heat flammable liquids or solids, hazardous substances or radioactive materials in any type of microwave oven.
  • Do not place metal items inside the microwave, including aluminum foil and plastic coated magnetic stirrer bars.
  • Do not modify the microwave in any way, including the removal of the grounding pin or change of the plug.
  • Never use a laboratory microwave for food preparation (or kitchen microwave for laboratory materials).
  • Do not heat sealed containers in a microwave. Even a loosened cap or lid poses a significant risk since microwaves can heat material so quickly that the container explodes either in the oven or shortly after removal.
  • Take care to avoid overheating liquids. It is possible to raise water to a temperature greater than normal boiling point; when this occurs, any disturbance to the liquid can trigger violent boiling that could result in severe burns.

Additional safety measures must be considered when using a microwave to melt agar:

  • Large amounts (e.g. 250 mL) of solidified agar should not be warmed in a microwave unless first chopped up with a sterile spatula. Not doing this can cause explosive vaporization in solid agar where vapor cannot escape.
  • Use loose fitting sterile foam plugs or loose 'Kimwipe" plugs, rather than just relying on loosely placed cap.
  • Ensure a good amount of headspace is available in the container above the material being heated.
  • Set the power and timings correctly. Do not overheat.
  • Thermal gloves and a face shield must be worn when removing a container from the microwave.
  • Care should be taken when placing the container on the bench as not to cause unnecessary disturbance of the agar, which may cause the agar to boil over.

Microwave ovens should not be utilized as a reaction accelerator in the chemical research field. A full risk assessment must be conducted first and may only be approved in laboratory-grade equipment.