Economists often urge the government to adopt policies that "internalize" an externality. Pigouvian taxation is revealed as not the only way to internalize an externality. Contracts, extended markets, and corrective taxation are equally capable of internalizing an externality. Pollution is an example for negative externality. Congestion is considered a negative externality by economists. In economic terms, pollution from fossil fuels is regarded as a negative externality. A positive externality is anything that causes an indirect benefit to individuals. There is an important conceptual distinction between a demerit good and a negative externality. Traffic congestion is an example of market failure that incorporates both non-excludability and externality. This represents a substantial potential negative externality. Pollution is a prime example of a market externality. Immunizations impose what is known as a positive consumer externality on society. Social accounting is in this sense closely related to the economic concept of externality. A tax shifts the marginal private cost curve up by the amount of the externality. Asian dust is an example of a negative externality on society.