Every time you refuel your vehicle, a small amount of vapors are released into the atmosphere.  These vapors include Volatile Organic Compounds or VOCs which are one of the classes of chemicals necessary to produce ozone.  Similar to mowing after 6:00 PM, if you can wait to refuel your vehicle, this gives the atmosphere a chance to disperse these vapors before the sun has a chance to ‘cook’ them. 

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www.tceq.state.tx.us