Negative externalities occur when production or consumption impose costs on third parties not involved in the market transaction. This leads to market failure as prices do not reflect the full social costs. Examples include pollution from factories imposing health costs, and noise pollution from airlines imposing nuisance costs. Taxes or regulations can be used to internalize these external costs and improve economic efficiency by aligning private and social costs. However, determining the appropriate tax level can be difficult and taxes may impact consumer welfare.