what does this symbol mean?

Hazard warning signs

The warning symbols for biological, radiation, electrical and laser hazards, and what each one identifies.

About hazard warning signs

These symbols warn of a hazard in a place or on equipment rather than in a chemical. There are seven: the biohazard symbol, the radiation trefoil, high voltage, the laser beam, non-ionizing radiation, a strong magnetic field and the EX sign for explosive atmospheres. In Great Britain they are drawn as warning signs, a black pictogram on a yellow triangle with black edging, the yellow at least half the sign’s area. Each page says what hazard a sign identifies; what to do in a marked area is set by the site’s own signs, instructions and trained staff.

Colours differ by country and regulator

In the United States each hazard has its own rules. OSHA’s biohazard labels are fluorescent orange or orange-red, and the Nuclear Regulatory Commission’s radiation symbol is magenta, purple or black on yellow. OSHA’s radio-frequency warning symbol is a red triangle above an upside-down black one, outlined in an aluminium colour, with “Warning—Radio-Frequency Radiation Hazard” in the upper triangle.

Two kinds of radiation

The British regulations give radioactive material and non-ionizing radiation separate warning triangles: the trefoil for the first, waves spreading from a mast for the second. Non-ionizing radiation here means electromagnetic fields such as radio waves. The Health and Safety Executive’s guide to electromagnetic fields at work lists broadcasting and TV antennas and radar and radio transmitters among the sources of high-frequency fields. It lists the main magnet of an MRI scanner among the sources of static magnetic fields, which have their own sign, a horseshoe magnet.

A sign made of letters

The EX sign has no picture. The EU’s ATEX workplace directive, 1999/92/EC, sets it out as a triangle with black letters on a yellow background with black edging, at the points of entry to places where an explosive atmosphere may occur: flammable gas, vapour, mist or dust mixed with air, in which combustion spreads to the whole mixture once it is ignited. Great Britain has the same sign in its Dangerous Substances and Explosive Atmospheres Regulations 2002.

Sources

  1. The Health and Safety (Safety Signs and Signals) Regulations 1996, Schedule 1, SI 1996/341, legislation.gov.uk (UK Government)
  2. Bloodborne pathogens (29 CFR 1910.1030), 29 CFR 1910.1030, U.S. Occupational Safety and Health Administration, 1910.1030(g)(1)(i)(C)
  3. Caution signs and posting requirements (10 CFR 20.1901–20.1902), 10 CFR 20.1901, U.S. Nuclear Regulatory Commission, 10 CFR 20.1901(a)
  4. Nonionizing radiation (29 CFR 1910.97), 29 CFR 1910.97, U.S. Occupational Safety and Health Administration, 1910.97(a)(3)(i)
  5. Electromagnetic fields at work: a guide to the Control of Electromagnetic Fields at Work Regulations 2016 (HSG281), HSG281, first edition, 2016, Health and Safety Executive, para 7 and Table 1
  6. Directive 1999/92/EC (ATEX workplace directive), Directive 1999/92/EC, European Union, via legislation.gov.uk, Articles 2 and 7(3), Annex III
  7. Dangerous substances and explosive atmospheres: Dangerous Substances and Explosive Atmospheres Regulations 2002, Approved Code of Practice and guidance (L138), L138, second edition, Health and Safety Executive, Schedule 4

Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research