This is a complex question which needs to account for your definition of acceptable, how your nation or other institution calculates its Air Quality Index, as well as the components of the pollution which must be blocked from entry into the respiratory tract.  There are four major components measured in an an Air Quality Index, all of which impact and individuals health. These are: Particulate matter (PM), Carbon monoxide (CO), Ozone (O3), Nitrogen dioxide (NO2) and Sulfur dioxide (SO2).

To add further complexity, the type of mask, duration-concentration dynamics of different pollutants, and individual personal health concerns must be considered for a perfectly relevant answer.

The United States Environmental Protection Agency recommends that individuals, with normal sensitivities and health, stay indoors beginning at an United States EPA AQI of 151.  It may be inferred that at Air Quality Index scores below 150, it is safe to be outside doing moderate physical activity, such as walking, without a mask.

For further information, read the World Health Organization's September 2021 publication: WHO global air quality guidelines: particulate matter (‎PM2.5 and PM10)‎, ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide
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