Reformulated gas was first sold in 1995. Its use is required by federal pollution-law amendments in some areas of the country, but it was picked as an anti-pollution strategy in others (such as Northern Kentucky).

The gas is refined to remove certain pollution-causing hydrocarbons. The hydrocarbons are particularly prone to evaporate in hot weather. Heat and sunlight combine to convert the hydrocarbons into ozone.

Additives also are put into reformulated gas to boost its octane, making the fuel burn more cleanly and also prevent car engines from knocking.

The additives for reformulated gas can include:

• Ethanol, an alcohol typically made from corn. Ethanol is used in the reformulated gas used in Northern Kentucky.

• MBTE, which is created from methanol (another kind of alcohol). MBTE is controversial because it is believed to cause cancer and it mixes with water (causing concerns when storage tanks leak and pollute groundwater or wells). Some states have banned MBTE.

The strictest formula of the cleaner-burning fuel is required in the summer because that's when the weather conditions needed to form ozone occur in most of the country.

The added cost of making reformulated gas is pennies a gallon. But since 2000, the standards for reformulated gas were tightened; there have been sharp price spikes each spring. Some experts say this is because refineries try to sell all the "winter" gas they can before switching over to the summer blend - and that sometimes causes shortages that drive up prices.

Source: Howstuffworks.com, Enquirer research