The index fell to 81.9, down from 88.6 the previous month and clearly below market expectations.
High living costs, especially high fuel prices, are fuelling the concern.
Also released on Tuesday were figures from the US Department of Labor showing that the number of job vacancies fell in August compared with the previous month. It also showed that fewer people were laid off and that the number of people who chose to quit their jobs remained at about the same level as the previous month.
