According to a U.S. Consumer Product Safety Commission (CPSC) report, engine driven tools (including electric generators) cause more non-fire CO deaths annually than gas furnaces, water heaters, and ovens combined. In fact, since 2005, there have been over 72% more CO deaths from engine driven tools than from heating systems.
Checking the peak CO levels after driving to inspections was certainly an eye-opener. Put it this way: now I know why people tend to get drowsy at the wheel.