Do you burn a lot of candles and do they have wire in the middle of the wicks?
Candles, purchased at The Gap and Banana Republic, were releasing soot and volatile organic compounds, such as benzene and toluene, when burned. The wicks had wire cores made of lead, a potent neurotoxin that was being released into the air.
Particle deposition occurs along thermal gradients. It may also occur on surfaces that are electrostatically charged. Such thermal gradients do occur near electrical outlets on exterior walls.
Or I found this (it's in regards to aluminum wiring):
Signs of overheating
More often, the burn-out happens unobserved. Look for signs of soot or scorching around or behind the face plate. Soot deposited at the connection slots of a receptacle is usually caused by a worn-out receptacle rather than by bad connections.
An overheated connection, especially at receptacle or switch terminals can cause the metal face plate screws to become very hot — even too hot to touch. The face plate itself will also be warm. A slight warming of the receptacle and plug is normal with a heavy load like a hair dryer or portable heater. No parts should ever become too hot to touch.
Well-behaved women seldom make history!