According to the federal regulations, research is eligible for exemption, if: All the subjects … of research activity described in…
"The research falls into one of six categories of research activity described in the regulations".
Researchs which are declared to be exempt are initially reviewed by IRB, but no more reviews would be conducted afterwards. First of all, if a project does not meet the requirements defined for a research with human subjects is directly excluded from the IRB review. Independently if the research qualifies or not as exempt it must follow some minimum ethical standards, such as: the research is minimal risk, the selection of participants is equitable, etc.