In recent years many individuals and organizations have advocatedthe use of several ventilation-based strategies to protect building occupantsfrom accidental and intentional releases of airborne chemical,biological and radiological (CBR) agents. For example, the protectionoffered by outdoor air filtration and air cleaning in combination withbuilding pressurization has been highlighted. However, many of theserecommendations have not considered the key role played by envelopeairtightness in determining the effectiveness of these strategies.