It's not likely to affect your emissions enough to fail. But all the time it's running cold it will be using more fuel.

The thermostat is a cheap part so will soon pay for itself in reduced fuel costs once replaced.

The thermostat is integral with the housing, so you have to change the complete plastic body. Not a difficult job, but a lot of bits have to be removed for access.

Don't think of buying a second-hand one (there's a lot of 'em on e-Bay) because the gasket fails with age. The rubber gasket swells and pushes on the internal plastic wall, which is too thin (design error, or deliberate?) to take the pressure and eventually (after eight years on mine) breaks so that you get a coolant leak from the side of the housing.