Smog producing ground-level ozone is a problem affecting the entire Greater Toronto Area. A major pathway for airborne pollutants flows from the upper Midwest United States and the Ohio River Valley and across southern Ontario and Toronto; key sources are coal-burning power-plants and vehicle engines. On "episode days" (O3 > 82 ppb), Whitchurch–Stouffville reaches its peak about one to two hours later than Toronto. Smog Advisory Alerts are issued by the Ministry of the Environment when smog conditions are expected to reach the poor category in Ontario. The Greater Toronto Area had 13 smog days in 2008, 29 in 2007, 11 in 2006, 48 in 2005.