Will I choke on my own CO2?

Ballpark estimate for closed room fears during summer

Although this blog has been talking mostly about computer systems, I would like to take the liberty of talking about HVAC (Heating, Ventilation and Air Conditioning) as it also is a system and more importantly, its understanding can help answer important questions and quell fears.

With summer in full swing, so is my split air-conditioner and it is common practice to use it with the doors and windows closed. However, I was recently asked a very reasonable question about it — “Will I choke on my own CO2?”. South Korea had wrestled with this idea for quite a bit and is a belief popularly known as “fan death” (nothing to do with heartbreak from shocking K-Pop news). To answer it, we can simply look at the amount of CO2 we exhale in a day and check if that takes the room air off-limits.

Before we start, we can assume the pressure in the room does not change by much as our ears would know first. Also, the cooler air does not translate to a large reduction in volume as well (we are going from perhaps 300 to 295 K) and that means less than 1% of volume decrease.

The human body releases around 6 litres of air per minute at rest. The exhaled air has around 4% of CO2 in it, translating to a 100x increase in concentration compared to the ambience! Thus, for an 8 hour period per person, we release

(8 hours) * (60 minutes/hour) * (6 litres/minute) * (0.04) = 115.2 liters

Typical bedroom sizes are around 200 square feet, and with a roof height of 10 feet, and this translates to 56 cubic meters.

A bedroom usually has 2 people in it, which means, they add 230 litres of CO2 for the night.

This translates to increasing the CO2 by 0.4%. Remember that we start with 0.04% of CO2 in ambience. The threshold for CO2 toxicity is at around 0.5%, after which one would start seeing symptoms, and that means, we would be skirting this danger at the end of our sleep!

Thus, as a very useful rule of thumb, “reset” your bedroom before going to sleep by ventilating the room for a while.

Shoutout to Prafulla Chandra A for inspiring this chain of thought.

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