Consuming carbon is dangerous in LARGE quantities so one ir two ieces of toast are nothing to worry about. If you eat burned toadt every day it could become a problem possibly. Its the folks who grill often and consume blackened carbonized meats frequently that have the most worries. A little here or there is nothing to freak over.
Overly grilled meats and burned foods produce carbon which is believed to be a risk factor. No one can say exactly what the exposure, magic number is. That said, it wouldn't be a bad idea to stop eating burnt toast.