Explain the statement: “Clean air and water have become increasingly scarce and valuable resources because they have been treated in the past as if they were free and unlimited in supply.” What methods might be used to internalize spillover costs?

Please provide the best answer for the statement.

Reasons why environmental pollution has become a major problem are that most pollution costs are external to the producer and user so that they have little incentive not to pollute. Rights to air and water have been traditionally treated as common-property resources with no price attached to their use, a situation called the “tragedy of the commons.” The excessive pollution that has resulted means that both clean air and water are now scarcer commodities. Negative externalizes from pollution can be internalized through the use of emission fees, markets for pollution rights, and, in some cases, private negotiations.