Fresh air is often considered healthier than indoor air, especially in areas with adequate ventilation. Maintaining health requires minimizing sources of indoor pollution and ensuring adequate ventilation to provide healthy indoor air quality.

No simple answer. Generally, carbon dioxide concentrations and other gaseous pollutants (e.g. formaldehyde) are higher indoors, assuming closed windows. In the summer, with air conditioning, the concentration of particulate matter is lower indoors especially if there is any smoke outdoors.

Thanks Pedro. I assume it would vary a lot with geography, seasons and other facotrs so I should be more specific. I had read in a few places that the external air microbiome was more diverse than indoors ! "Diversity and compositional differences of the airborne microbiome in a biophilic indoor environment" https://www.nature.com/articles/s41598-023-34928-9