09/11/2026
My bestie told me she’s been waking up with headaches randomly but they go away after a few hours of being awake. Naturally, I say “your house is too tight”. She asked what that meant and how to test, so here is what we found.
Her indoor CO₂ measured 1,199 ppm.
So what does that actually mean?
We breathe out carbon dioxide all day—and all night. In a bedroom with the door and windows closed, especially in a newer or tighter home, CO₂ can accumulate when there isn’t enough fresh outdoor air coming in.
CO₂ is one of the tools we can use to evaluate how well a space is being ventilated.
There isn’t one universally accepted “toxic at 1,000 ppm” cutoff. In fact, ASHRAE specifically says its standards do not establish 1,000 ppm as a health-based indoor CO₂ limit.
But that doesn’t mean I ignore a reading like this.
🌿 Outdoor CO₂ is generally much lower than what we see in a poorly ventilated occupied room.
🏠 Building Biology guidance demonstrates how CO₂ can climb dramatically as air-exchange rates decrease—for example, its Healthy Home Standard shows an example reaching >1,500 ppm at 1 air change/hour and >3,000 ppm at only 0.3 ACH with three occupants.
🏛️ Public-health guidance has historically used ~1,000 ppm as a practical indication that ventilation should be investigated, rather than as a toxicity threshold.
🧠 And research gets interesting: a controlled study found statistically significant decreases in several measures of decision-making performance at 1,000 ppm compared with 600 ppm, although subsequent research has found mixed results about whether CO₂ itself—or the other pollutants that accumulate when ventilation is poor—is responsible.
Continued in comments 👇🏻
