Why would a fire extinguisher class icon have a red diagonal line through it?

A fire extinguisher class icon featuring a red diagonal line indicates that using that particular type of extinguisher on that class of fire could introduce additional risks. This sign serves as a critical visual warning to alert users that applying that extinguisher could lead to more dangerous or hazardous conditions rather than effectively controlling or extinguishing the fire.

For example, using a water-based extinguisher on a grease fire can cause the flames to spread, potentially leading to a larger fire and injury. Therefore, understanding these symbols is crucial for fire safety and ensuring that firefighters and civilians alike utilize the right equipment for specific fire situations to mitigate risks effectively.