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Posted by on Apr 20, 2012 in Friday Fiction Facts, Science, Writing | 17 comments

Friday Fiction Facts: Trapped in an airtight room!

Welcome to the first edition of  Friday Fiction Facts – that is, 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.


  1. This is so great! I think it’s one of my favorites! Hey, come on over to Motherlode later on today – I go live around 3ish…a fun topic – teachers stay on for an average of 4.5 year and cite “issues with parents” as one of the major reasons they leave the profession. I offer up an etiquette guide for teachers and parents…

  2. Brilliant, Kim! Next time I lock a character in a room, I’m coming straight here first. And let’s face it, it’ll probably happen, sooner or later. 😉

  3. I guess having the character hold his breath wouldn’t work, eh?

  4. Hey, you three should know each other. All teachers, all parents, all incredibly good writers with a strong focus on children. If you’re not following each other on Twitter, go do that. Go on! I’ll wait.

    Thanks everyone for stopping by -K~

  5. This sounds like an excellent new series you’ve cooked up!

    I’m curious about why the carbon dioxide concentration in normal air is so low. If we take in a lungful of air and then breathe it out, how much of that lungful’s oxygen was NOT converted to CO2? I always thought it would be a more 1-1 relationship as we take in what plants give off, and the plants take in what we give off.

  6. That’s a really good question Christina.

    Normal air is about 21% oxygen. Of that, we extract only about 3-6%. So the air we exhale is about 15-18% oxygen. Almost as much as we start with.

    Normal air is about .03% CO2. Exhaled air has a CO2 concentration which is much greater – more like 2.5-6%. That’s because CO2 is a by-product of cell metabolism and we have to get it out of our bodies. The only way to do that is to exhale it.

    So in the case of our trapped main character, she is exhaling almost all of her oxygen with every breath. So there is plenty of that to sustain her. But, she is adding to the concentration of CO2 with every breath so that’s going to build up over time.

    However, in an open system (outdoors), that CO2 has a place to go — plants take it up. Through photosynthesis they break up the CO2 molecule, keep the carbon (as plant material) and release the spare oxygen back into the air.

    So both plants and animals are releasing oxygen, in large quantities, but the plants are hanging on to the carbon, so we don’t get that back into the air until the plants decay, animals eat them (and exhale the CO2) or we burn them ..either as wood or as fossil fuels. (There are few other ways as well; volcanoes for instance).

    Since we’re really talking about the carbon cycle, you can read more here:

    Does that make sense and answer your question? -K~

  7. I <3 this entry and look forward to reading more!

  8. Math is not my strength…how long would a person last in an old, not running, deep freezer? My character needs to know! thanks

  9. My main character is locked in a box but it lets in cold air and has a tiny slit that opens where her captors let her drink water. Mine is more of a weakening through starvation thing. I was wondering how long the stress of hunger and lack of muscle movement would work. Is hysteria and option here? Got any ideas?

  10. Interesting read, however it is based on several incorrect assumptions. First that the toxic level of co2 is 3%. It’s not. One can easily withstand 4% co2 for weeks at a time. Likely one could continue to survive until co2 is somewhere between 5-10%

    Also a person does not typically produce 1.7 cubic feet of co2 per hour.

    Average tidal volume is 500ml, respiration rate is 12-24 per minute, which means the average person, at rest will breath about 6-12 liters of air every minute. Each breath will expire 4% co2. Assuming 12 liters per minute, a person would produce .48 liters of co2 every minute, or 28.8 liters per hour, which is about 1 cubic foot…

    Following your equation, assuming the toxic level of co2 is 5%, which is probably actually lower than it is, the character has 50 hours until they have produces a toxic level of co2.

  11. so if you went in there you would die 🙁


  13. Ever wonder just how much the production of CO from our modern gas furnaces that now EXIT THRU THE BASEMENT WALL AT GROUND LEVEL and the industrial uses contribute to still born babies?

    • Hi, when I was 22yrs old, my friend luck me in a mini bus full of noodles box, just for 10mins I was shouting but no body open the car door for me. I am now 33yrs and I always has that fear of not getting enough air when I am in d bus,airplane,lock room, I resently went for an ear test for my new job, during the ear test made me to seat on a chair inside a small closed cubord, I was fine for 3mins and my mind was telling am not getting enough oxygen here then I be came uncomfortable and push the door open. Am tired of this because if my mind did not remind me that am not getting enough oxygen I will not panic or shake,I am fin but once my mind tells me that then I will be looking for more oxygen. Please what will I do, I need to get this out of my brain, I need solutions pleas, I can not enter bus or airplane because of all this. Thanks cyprian

      • Hi Cyprian,

        That sounds very frightening! But you are not alone. The fear of being enclosed in tight spaces and running out of air is very common. It is called “claustrophobia” and millions of people suffer from it. For some people it’s just a fear of very small places like closets, so they are able to avoid those places.

        For others, like you, it interferes with their lives because they are prevented from traveling in vehicles, camping in tents, and even sitting in dark movie theatres and getting proper medical care like hearing tests or MRIs.

        The good news is that claustrophobia can be treated with counseling or therapy. Tell your doctor about your fear and he or she can help you find a therapist who will teach you how to overcome your anxiety. Also your doctor may be able to prescribe medication that will help calm you if you must travel or have a medical procedure like a hearing test.

        You can read more about claustrophobia here:

        Thank you for writing Cyprian and best of luck to you,


  14. Hey, Kim, awesome article! I’ve been trolling around for a question I have about this situation, and I think you might be the best person to ask. I What if you had some sort of device that was like the opposite of a a respirator, that you could use to suck up your exhalations into a tank? I figure that might save you from the Co2 poisoning. How much time do you think could you buy yourself before you died of straight-up oxygen deprivation? Could the same effect be achieved with exhaling into a big plastic bag or something, or would it start filling up your space too much? Thanks!!!

  15. How long would a child (around 8yrs old) survive if locked in a 4 cubic foot container? I would really appreciate help with this!


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