Biogenic VOCs can either reduce ozone pollution or create it, depending on what else is brewing in the atmosphere. Bright, balmy conditions are best for VOC production, which trees may perform to protect against thermal stress. On a 95-degree day, a forest of 10,000 poplar trees can crank out more than three kilograms of isoprene an hour, the equivalent vapor emission of spilling three gallons of gasoline. Isoprene forms ozone when it mingles with NOx in the atmosphere. But if NOx levels are low, it destroys ozone.