As with most EVAP faults, I would say this is low on the severity scale. The entire system was mostly designed to decrease emissions to the atmosphere. Obviously it does a lot more in the meantime but all that being said, really the only thing negatively affected by this fault is the atmosphere. At the moment, I cannot think of any EVAP system problem that would be detrimental to the overall safety of the vehicle. This does NOT mean you can continue to drive the vehicle day in day out without addressing the issue. One problem always leads to another if left unsolved for too long.