Regulators should tighten lending rules for home mortgages in the U.S. and the government is taking on a lot of risk by guaranteeing risky loans, Federal Deposit Insurance Corporation Chairman Shelia Bair warned. "I think we should all be concerned about the type of exposure that the government is taking on through guaranteeing so many mortgages right now, and make sure that we do have some prudent underwriting standards, and especially documented ability to repay," Bair told CNBC Wednesday.