Opportunity cost is the cost related to the 2nd best choice available to someone who has picked among several mutually exclusive choices.
Opportunity cost is assesssed in not only monetary or material terms, but also in terms of anything which is of value. Monetary and financial costs, the real cost of output forgone, lost time, pleasure, and other benefit provides utility are all considered opportunity costs.
In environmental protection, the opportunity cost is also applicable. This has been demonstrated in the legislation that required the carcinogenic aromatics (mainly reformate) to be largely eliminated from gasoline. Unfortunately, this required refineries to install equipment at a cost of hundreds of millions of dollars - and pass the cost to the consumer. The absolute number of cancer cases following from exposure to gasoline, however, is low, estimated a few cases per year in the U.S. Thus, the decision to legally require less aromatics has been criticized on the grounds of opportunity cost: the hundreds of millions spent on process redesign could have been spent on other, more fruitful ways of reducing deaths caused by cancer or automobiles. These actions (or strictly, the best one of them) are the opportunity cost of reduction of aromatics in gasoline.