(Combined Sources) Massachusetts had just three unhealthy ozone days during the 2020 monitoring season that just ended, the fewest since monitoring began in the 1970s. State environmental officials say all three unhealthy air days occurred in late July and affected areas along the South Coast, Cape Cod and the Islands. The state Department of Environmental Protection said the rest of the state experienced no unhealthy air days during the ozone monitoring season, which runs from March through September. Ozone is produced when heat and sunlight react with air pollutants and can cause respiratory problems.