— middle-aged people alive today experienced a 35% increase in average ambient atmospheric carbon dioxide concentration within their lifetimes[0], and

— ambient atmospheric carbon dioxide concentration today has apparently never been this high since Miocene[1] (15 megayears ago). It blew past the last relative peak, from around 300 kiloyears ago, around World War I and Russian Revolution[2] and is skyrocketing since then.

This is not a hypothetical downstream effect from global warming or sea level rise, it’s what all of us breathe. When talking about indoor spaces, official recommendations are always to keep it as close to outdoor air as possible. However, “outdoor air” is a moving target: give it another 35% increase, and planet average will be reach 600 ppm. Meanwhile, 1000 ppm to be a safe limit[3] for round-the-clock exposure by an average human.

[0] https://www.climate.gov/news-features/understanding-climate/...

[1] https://pubmed.ncbi.nlm.nih.gov/10963587/

[2] https://nsidc.org/learn/ask-scientist/core-climate-history

[3] https://www.canada.ca/en/health-canada/services/publications...