There are many possible solutions to the acid rain problem. One solution is to use fuels that burn more cleanly. Another solution is to burn coal more efficiently. Power plant and factory chimneys can be fitted with scrubbers that release 90-95% sulfur free smoke and also produces sludge from which gypsum, a building material, can be produced. This problem can also be solved by enforcing tight emission standards on vehicles that burn gas and diesel. One other solution is fitting devices such as a catalytic converter into car exhaust pipes to minimize the amount of sulfur dioxide in car exhaust.