We also found that park visits fall on high ozone days – especially during summer and fall, when peak ozone levels typically occur.

By the year 2000, ozone levels in national parks were, on average, very similar to those in metropolitan areas.

This response was most pronounced during summer and fall, when both visitation and average ozone levels are highest.

They discovered that the ozone levels had dropped by 40 per cent over 30 years.

Since the early 2000s, ozone levels in both national parks and metropolitan areas have improved.