Gases:
* Helium (He): Helium is the most well-known example. It's incredibly light and doesn't burn, which is why it's used in balloons and airships.
* Hydrogen (H2): While technically less dense than air, hydrogen is highly flammable and should be handled with extreme caution.
* Neon (Ne): Neon is also less dense than air and non-flammable, giving it its characteristic glow in neon signs.
* Argon (Ar): Argon is another inert gas that is less dense than air and doesn't burn.
Other:
* Some foams: Certain foams are designed to be less dense than air and are non-flammable. This can be used for insulation or buoyancy applications.
Important Note: While these substances are less dense than air, their density can vary based on temperature and pressure.
