Final answer:
The APA reported surveys showing significant levels of stress and impacts on health but a specific percentage of adult Americans experiencing adverse health effects from stress as per the excerpts is not directly provided.
Step-by-step explanation:
The specific percentage of adult Americans who suffer adverse health effects due to stress, as reported by the American Psychological Association (APA), is not directly provided in the text excerpts. However, the related information suggests that stress can contribute to various psychological disorders and chronic health problems, and that recent surveys reflect significant stress levels in the American population. Although a direct statistic isn't provided, the APA has conducted surveys revealing trends in stress, for instance, Cohen & Janicki-Deverts (2012) show different demographic groups experiencing varying levels of stress and its impact on health over time.