The little chart below is from the 2014 EPA "Risk Assessment" which evaluated the health risks of dirty air under various possible ozone standards. The report is at summary of EPA ozone risk assessment
EPA experts sought to apply several hypothetical standards to the 12 major specific cities noted below.
As you can see, more people risk death at an ozone level of 70 than at lower levels. (EPA's assessment went down as far as 60 -- the standard supported by public health groups.)
The report examines other health risks as well. In all cases, an ozone standard of 70 provides less public health protection than would a more restrictive standard.