W with a line through it (water reactive)
On an NFPA 704 fire diamond, a W with a line through it, in the white section at the bottom, marks a material that reacts unusually with water, violently or explosively.
How to read it
It is one of the three special hazard symbols of the NFPA 704 fire diamond. NFPA's FAQ describes it as a caution about the use of water in firefighting or spill control, and CAMEO Chemicals also calls it “No water”. The line through the letter is part of the symbol. The details are on the product's safety data sheet: the NFPA and OSHA quick card looks for special hazards in sections 9, 10 and 11.
Also searched as: crossed out w, nfpa w, water reactive symbol.
These explain what a rating on the diamond means. For handling, storage and first aid, refer to the product’s label and safety data sheet.
Which materials carry it
Fire Engineering names lithium, magnesium, calcium and aluminium as common water-reactive substances, and counts the combustible metals of fire class D among them. A Department of Energy laboratory guide's worked example rates calcium 3 for health, 1 for flammability and 2 for instability, with the W.
Fire Engineering traces the diamond itself to a 1959 fire in Charlotte, North Carolina, where a vat firefighters took for kerosene held metallic sodium, which reacted violently to their efforts to put the fire out.
Look-alikes
On OSHA and GHS container labels, substances that give off flammable gas in contact with water take the flame pictogram and a hazard statement such as “In contact with water releases flammable gas”. The NFPA and OSHA quick card shows both on one example product: the diamond with its W, and the OSHA label with that statement.
The yellow instability number once covered reactions with water too: in the University of Florida's chart of the 1996 edition, a 2 included materials that may react violently with water.
More NFPA 704 fire diamond
Browse symbols drawn with letters and numbers or lines.
Sources
- Frequently Asked Questions on NFPA 704, National Fire Protection Association, published by the Cambridge Fire Department (Massachusetts), Question 5
- NFPA Diamond (CAMEO Chemicals help), NOAA Office of Response and Restoration
- Understanding NFPA 704 Placarding Is Critical to Life Safety of First Responders, Brian Daboul, 29 January 2024, Fire Engineering
- Hazardous Materials Facility Markings, AWR-358 course, Center for Domestic Preparedness, U.S. Department of Homeland Security
- Evaluating Hazardous Materials for NFPA 704 Diamond Ratings, WIPP Information Repository, Supplemental Information, U.S. Department of Energy, Waste Isolation Pilot Plant
- Comparison of NFPA 704 and HazCom 2012 Labels (Quick Card), National Fire Protection Association and U.S. Occupational Safety and Health Administration
- Hazardous Material Code Identification (NFPA 704, 1996 Edition), University of Florida Environmental Health & Safety
- Allocation of Label Elements (29 CFR 1910.1200, Appendix C), 29 CFR 1910.1200 Appendix C, U.S. Occupational Safety and Health Administration, C.4.25
Last reviewed September 30, 2026 by Rory Hansen. How we research