Unless you're sanding old paint, airborne lead is not likely to be a problem. (However, lead was in solder until the late '80s, so run your water taps a minute after they have been off for three hours.)

But large amounts of airborne moulds, if they're present, could be a problem due to the Stack Effect. Warm air rises, so in the winter especially, up to half the air you're breathing comes from your basement.

sealing off the basement.

Unless this includes foaming the entire basement ceiling and somehow sealing off all the supply and return pipes and the furnace itself, you're not truly sealing off the basement.

Keeping your basement's relative humidity below 50% with a dehumidifier should be all you need to keep mould growth in check. Properly sealing and insulating the rim joists will help greatly, too, by cutting down the infiltration of warm humid air in summer and preventing condensation in winter.

Here is a link that might be useful: Stack Effect

This post was edited by worthy on Sat, Mar 9, 13 at 15:30