An accidental or intentional outdoor release of pollutants can produce a hazardous plume, potentially contaminating large portions of a metropolitan area as it disperses downwind. To minimize health consequences on the populace, government and research organizations often recommend sheltering in place when evacuation is impractical. Some reports also recommend "hardening" an indoor shelter, for example by applying duct tape to prevent leakage into a bathroom. However, few studies have quantified the perceived beneficial effects of sheltering and hardening, or examined the limits of their applicability. In this paper, we examine how sheltering and hardening might reduce exposure levels under different building and meteorological conditions (e.g., wind direction). We predict concentrations and exposure levels for several conditions, and discuss the net benefits from several sheltering and hardening options.