Double check human CO2 thresholds

[ezio-melotti]

The CO2 thresholds we are currently using are:

>=0.1%: complaints of stiffness and odors
>=0.25%: general drowsiness
>=0.5%: adverse health effects
>=1%: termination

According to the PRD, the source for these values (https://www.quora.com/How-can-carbon-dioxide-be-toxic, 4th answer by John Flavin) says:

Concentration tables were established to insure proper air circulation and to prevent any possible negative effects to people in their homes or the workplace.

• normal outdoor level: 350 - 450 ppm
• acceptable levels: < 600 ppm
• complaints of stiffness and odors: 600 - 1000 ppm
• ASHRAE and OSHA standards: 1000 ppm
• general drowsiness: 1000 - 2500 ppm
• adverse health effects may be expected: 2500 - 5000 ppm
• maximum allowed concentration within a 8 hour working period: 5000 - 10000 ppm
• maximum allowed concentration within a 15 minute working period: 30000 ppm

These values don't seem to be sourced, and our thresholds also appear to be shifted compared to this list.

I did a quick search and found:

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5380556/ which says:

  CO2 toxicity in humans: Carbon dioxide at low concentration has little, if any, toxicological effects. At higher concentrations (>5%), it causes the development of hypercapnia and respiratory acidosis. Severe acidosis increases the effects of parasympathetic nervous activity, possibly by interfering the hydrolysis of acetylcholine by acetylcholinesterase, resulting in a depression of the respiration and the circulation [6]. Concentrations of more than 10% carbon dioxide may cause convulsions, coma, and death [1, 15]. CO2 levels of more than 30% act rapidly leading to loss of consciousness in seconds.

• https://en.wikipedia.org/wiki/Hypercapnia#Tolerance

  %CO2 in inspired air	Duration	Major limitation
  0.028	lifetime	atmosphere, year 1780
  0.04	lifetime	current atmosphere
  0.5	lifetime	no detectable limitations (Note: refer to modern research in Carbon dioxide#Below 1% which shows measurable effects below 1%.)
  1.0	lifetime
  1.5	> 1 month	mild respiratory stimulation
  2.0	> 1 month
  2.5	> 1 month
  3.0	> 1 month	moderate respiratory stimulation
  3.5	> 1 week
  4.0	> 1 week	moderate respiratory stimulation, exaggerated respiratory response to exercise
  4.5	> 8 hours
  5.0	> 4 hours	prominent respiratory stimulus, exaggerated respiratory response to exercise
  5.5	> 1 hours
  6.0	> 0.5 hours	prominent respiratory stimulus, exaggerated respiratory response to exercise, beginnings of mental confusion
  6.5	> 0.25 hours
  7.0	> 0.1 hours	limitation by dyspnea and mental confusion

• https://www.epa.gov/sites/default/files/2015-06/documents/co2appendixb.pdf (this might be the same source used by the original Quora answer)

  Table B-1. Acute Health Effects of High Concentrations of Carbon Dioxide

  Carbon Dioxide Concentration (Percent)	Time	Effects
  17 - 30	Within 1 minute	Loss of controlled and purposeful activity, unconsciousness, convulsions, coma, death
  >10 – 15	1 minute to several minutes	Dizziness, drowsiness, severe muscle twitching, unconsciousness
  7 – 10	Few minutes	Unconsciousness, near unconsciousness
  7 – 10	1.5 minutes to 1 hour	Headache, increased heart rate, shortness of breath, dizziness, sweating, rapid breathing
  6	1 – 2 minutes	Hearing and visual disturbances
  6	≤16 minutes	Headache, dyspnea
  6	Several hours	Tremors
  4 – 5	Within a few minutes	Headache, dizziness, increased blood pressure, uncomfortable dyspnea
  3	1 hour	Mild headache, sweating, and dyspnea at rest
  2	Several hours	Headache, dyspnea upon mild exertion

  and it also contains a table with the tolerance time:

  Table B-2. Physiological Tolerance Time for Various Carbon Dioxide Concentrations

  Concentration of Carbon Dioxide in Air (percent by Volume)	Maximum Exposure Limit (Minutes)
  0.5	indefinite
  1.0	indefinite
  1.5	480
  2.0	60
  3.0	20
  4.0	10
  5.0	7
  6.0	5
  7.0	Less than 3

• https://en.wikipedia.org/wiki/Carbon_dioxide#/media/File:Main_symptoms_of_carbon_dioxide_toxicity.svg

I think we should revise and update these values, both in the PRD and in the code. We might want to take into account the pressure/temperature/humidity too, since they will probably be different in the habitat and might affect the results.

I also recommend we go through the other values (e.g. for oxygen) to see if we have other dubiously sourced data to double check.