A retrospective study by NIOSH found a significantly higher death rate from atherosclerotic heart disease for New York City tunnel officers compared with the expected rate for the New York City population. A Cox proportional hazards model which examined the risk of atherosclerotic heart disease for tunnel officers compared to bridge officers found a similar pattern. The Cox model also showed a significant decline in the risk of atherosclerotic heart disease among tunnel officers, when compared to the less-exposed bridge officers, after cessation of exposure with most of the risk dissipating within as little as five years.