Final answer:
Increased air pollution is not a cause of the decline in the overall mortality rate in the United States; rather, improved healthcare access, medical technology, and reduced smoking are the key contributing factors.
Step-by-step explanation:
The overall mortality rate in the United States decreased by more than 25 percent between 1981 and 2018. The factors contributing to this decline are improved healthcare access, advancements in medical technology, and the decreased prevalence of smoking. However, the option of increased air pollution is not one of the causes for the decline in the overall mortality rate. In fact, increased air pollution would likely have a detrimental effect on health outcomes and potentially increase mortality rates.
Among the many factors that contribute to the decrease in mortality rate are enhanced public health measures like clean water and sanitation, public behavior changes like the reduction in smoking, and medical leaps such as the development of immunizations. It is these improvements in public health, behavior, and medicine that have significantly reduced death rates and allowed for the substantial increase in global life expectancy over the past two centuries.