The New York Times has an estimate:
Unlike the ignition defect in General Motors vehicles that caused at least 124 people to die in car crashes, Volkswagen pollution is harder to link to individual deaths. But it is clear to public health researchers that the air pollutants the cars illegally emitted damage health, and they have formulas for the number of lives lost from excess pollution in general. Indeed, the Environmental Protection Agency uses its own estimates of the health effects of air pollution to create its regulations of what’s allowed. After consulting with several experts in modeling the health effects of air pollutants, we calculated a death toll in the United States that, at its upper range, isn’t far off from that caused by the G.M. defect.
The full story is here.