Lead-based paint or radon gas

What is lead-based paint or radon gas, and where is it likely to be found within a residence?
What are the health hazards and risk of lead-based paint or radon gas?
How is a building assessed for lead-based paint or radon gas risk? What procedures should be followed to determine if the hazard exists?
If there is a risk within a home or building, how are the risks mitigated or removed?