In the early morning hours of November 22, 2006, an explosion and fire destroyed a chemical manufacturing facility operated by CAI Inc. and Arnel Company in Danvers, Massachusetts. The blast damaged or destroyed more than 90 nearby homes and was felt across a wide area of northeastern Massachusetts. No workers were killed because the explosion occurred during a low-staffing overnight shift.

The investigation found that a mixing tank containing heptane and alcohol solvents had overheated overnight. The facility's ventilation system was routinely shut off at the end of each working day as a cost-saving measure. Without ventilation, flammable solvent vapors accumulated throughout the facility building until reaching explosive concentrations. The overheating tank provided both the vapor source and the likely ignition source.

The CSB found that Arnel's routine practice of shutting off ventilation at night created a predictably dangerous condition that had never been evaluated in a formal hazard analysis. The incident demonstrates a fundamental principle: ventilation is a safety-critical engineering control in any facility handling flammable solvents, not an optional comfort system that can be turned off to reduce operating costs.