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KIRKSVILLE, MO -- We all know gas prices go up during the summer months and go down in the winter. Some of this has to do with demand but a lot of it has to do with the type of gasoline being produced. For Friday’s Facebook Story of The Day, our viewers wanted to know more on summer blend gasoline versus winter blend gasoline.

During the summer, pollution is a frequent concern due to increased levels of smog and ozone. Summer heat can boost the formation of ozone and trap pollutants in the lower atmosphere.

Because of this, summer blend has to meet strict standards that are set up by the Environmental Protection Agency, or EPA, on how gasoline burns. Summer blend must be cleaner burning to help with air quality.

The main difference between the two has to do with the Reid Vapor Pressure of the fuel. RVP relates to the volatility of a gasoline. The more volatile a gasoline, the more likely it will evaporate as the temperatures rises; evaporated gasoline contributes to unhealthy ozone and smog levels. Summer gasoline has a low RVP and is less likely to evaporate when compared to the high RVP winter grade.

The Energy Information Administration says the switch between the two fuels happens twice a year, once in the fall (winter blend) and again in the spring (summer blend).

“Summer gasoline has to be sold starting June 1 and that goes through September 15. Then we start transitioning to winter gasoline. There are a few different phases. It’s not just a light switch and we are undergoing that transition right now from summer to winter gas. We will finally get there in about a month,” GasBuddy.com Senior Petroleum Analyst Patrick DeHaan said.

In many large cities as well as California and New England, the EPA requires the use of reformulated summer and winter blend gasoline’s. These reformulated blends contain oxygenates that lower RVP and other toxic chemicals even further than conventional gasoline.

And until winter blend gasoline is fully in place, prices are likely to continuing falling but aren’t the only reason we are seeing lower prices at the pumps.

“Demand is dropping for gasoline. We are getting into the cooler months. People aren’t driving as much whether it is vacation or not boating as much. That all means that demand has dropped and allowed supply to increase thus resulting in some of the decreases we have been seeing,” said DeHaan.

Because of the seasonal fuel program, the EPA claims roughly 75 million Americans now breather cleaner air.

States and even cities have the right to impose stricter gasoline qualities and often contribute to the fluctuation in prices from place to place around the country.