Several efforts are now underway to develop wireless technologies based on cognitive radios (CR) that can operate in the television (TV) band white space. One such example is the IEEE 802.22 Working Group (WG), which is formulating the first worldwide CR-based standard to operate in the TV bands. A mandatory requirement for any wireless technology operating in these bands is that it shall not cause harmful interfere with the passive TV receivers. In this paper we perform a comprehensive analysis of the effect of cumulative interference generated by the IEEE 802.22 system using circular cells and show how geometry plays an important role in the keep-out region. We then generalize our results and derive a generic formula to calculate the keep-out region for a typical deployment and for a variety of technologies.