Welcome to the first edition of  Friday Fiction Facts – sciency things that fiction writers need to know. This week we visit the “airtight room” – a staple of good thriller, mystery, and crime stories.

Your main character is trapped! She’s in a 10x10x10 foot room buried deep beneath the ground. Or maybe she’s tied up in an old walk-in freezer, the door sealed tight. Or perhaps your hero and sidekick are in a space capsule or an undersea rover; all systems have failed! Quick, how long do they have before they run out of oxygen?

Here are five things fiction writers need to know about The Airtight Room.

1. It’s not the Oxygen, it’s the CO2

The issue of suffocating in an enclosed space is not one of running out of oxygen; it’s one of being poisoned by carbon dioxide — CO2.  CO2 becomes mildly toxic at a concentration of 1%. (Normal atmospheric concentration is 0.036 %) A concentration of 10% can cause respiratory paralysis and death within minutes.

2. The more you breathe, the worse it gets

How fast the CO2 level builds depends on how fast you produce it. This would be related to how fast you are breathing. At rest you would exhale much less than if you were exercising.  A moderately active or stressed person produces about 1.7 cubic feet of CO2 per hour.

3. How long? There’s an equation for this!

Assuming a concentration of 3% CO2 is the highest safe limit, you can calculate how long a given number of people can stay in a given sized space before toxic levels of CO2 build up —

T = Number of hours before CO2 reaches toxic levels and your character(s) could die.

  (Volume of air inside the room in cu ft) x (3% or 0.03)
T = ---------------------------------------------------------
  (Number of people) x (one person's hourly production of CO2 in cu ft)


So, let’s see how that works for one person in a 10x10x10 (1000 cubic foot) space.

     (1000) x (.03)        30
T=   ------------     =   ----   = 17.64 HOURS
     (1) X (1.7)          1.7


4. Your main character will feel the pain

But, while your character may have 17 hours to get out of that room, keep in mind, she is not going to be much help when it comes to figuring out how to escape. The effects of CO2 buildup are going to take their toll fairly quickly so, unless it happens right at the beginning, there will be no clever MacGyver inventions or elaborate escape plans being hatched. Rescue is going to have to come from the outside.



Hour by hour (based on 1 person in a 1000 cu ft room), here is what your character is going to feel over time:

30 mins .1 % Slight Headache
6 Hours 1 % Slight increase in respiratory rate, hardly noticeable; feeling hot and clammy, inability to concentrate, fatigue, anxiety, clumsiness and loss of energy, inability to control limbs reliably (“jelly knees”).
12 Hours 2 % Breathing rate will be 50% faster, headache after a few hours at this level, tiredness.
18 Hours 3 % Breathing rate doubles, panting, dizziness, severe headache, vision disturbances (sparks, stars, speckles), reduction in night-vision, blood pressure increase, may affect hearing; Prolonged exposure to this concentration may cause extreme sluggishness but usually not death
24 Hours 4-5% Immediately dangerous. Breathing 4x normal rate, feeling unable to catch breath, severe headache, choking feeling and unconsciousness within 30 minutes. May cause permanent side effects. Prolonged exposure can cause death.
30 Hours > 5% Extreme rapid breathing, choking sensation, tinnitus, impaired vision, confusion;  At 10%, unconsciousness and death within a few minutes.


Note that times are rounded and very generous. In reality, the adverse effects will be compounded.  In other words, at 24 hours, your character has already been exposed to increasingly high CO2 levels for a full day. Also, his or her rapid breathing will have created CO2 faster that 1.7 cu ft/hr that we used for the calculation.  This is why rescue will need to take place well before 18 hours.

5. It’s not over when it’s over

Recovery is not immediate. Your newly rescued main character will not be running away from bad guys, fighting off killer zombies, or swimming 200 meters back to the boat.  Exposure to 2% CO2 concentrations for hours can result in loss of energy, headaches, and feeling of being run-down that can take days to go away.  Exposure to 4% or higher can damage the body and cause long-term or even permanent side effects.

So, my advice is to get your main character out of that room well before CO2 levels become dangerous. Otherwise, he or she may not be around for your sequel.

Got a question or an idea for Friday Fiction Facts? Let me know.

*Disclosure:  This post is based (and partially copied) from an answer to a query I wrote on Google Answers in 2003.  The work is my own.