Source? The closest I could find is [1], which says 10 μg/m3 annual mean and 25 μg/m3 24-hour mean. It is for outdoor levels, but I can't imagine why the same chemical would be safer to breathe depending on whether you are inside or not as you breathe it.

[1] https://www.who.int/news-room/fact-sheets/detail/ambient-(ou...