The swelling of intumescent materials is triggered by what condition?

Intumescent materials are designed to provide fire protection by swelling in response to heat. When exposed to high temperatures during a fire, these materials undergo a chemical reaction that causes them to expand significantly, creating a barrier that helps to insulate and shield surrounding structures from flames and heat. This swelling is an essential feature, as it increases the material's volume and reduces the passage of heat, thereby enhancing fire resistance and improving overall safety in buildings.

The other conditions listed, such as moisture in the air, cold temperatures, and mechanical pressure, do not trigger the swelling effect of intumescent materials. Intumescent materials specifically rely on high temperatures to activate their fire-resisting properties, making heat from a fire the correct trigger for their swelling action.