Oxygen LevelOSHA defines “safe” breathing air to have between 19.5 and 23.0% oxygen (O2) by volume. This band is narrow for regulatory reasons, and it can be stretched a bit lower if the gasses displacing the oxygen are not dangerous and a bit higher if you control sources of ignition.
I know all of you are trained in CPR and mouth-to-mouth resuscitation. Do you realize that the air you're exhaling into that unconscious person is about 16% O2? That falls below the OSHA standards, but is enough to keep a person alive. You'll likely pass out at around 10-15%, which is why I use 15% as a lower limit. Once you've passed out, you are no longer functional and are as good as dead in a crisis/emergency situation. Physical condition and various health issues are going to determine your personal threshold.
On the upper side, you can breathe 100% oxygen for short periods of time (hours) without permanent damage. The major problem with any concentration over 25% is the increased risk of fire. Oxygen itself doesn't burn, but it combines with other substances to create fire. Oxygen enriched environments require special attention to the choices of clothing, electrical equipment, and other things that may ignite or cause a spark.
So, how do you measure the oxygen level in a room? There are various detectors on the market that use electrochemical sensors to measure specific chemical concentrations in the air, but they're not cheap. A typical 4-gas detector will measure O2, Carbon Monoxide (CO), Hydrogen Sulfide (H2S), and combustible gasses, but will cost between $500-1000 and require frequent calibration. I found the test results from an experiment that gives a much cheaper method: a candle will go out if the O2 drops below about 18%.
For years, the US Coast Guard used “flame safety lamps” to check for sufficient O2 in holds and other shipboard spaces, since a flame can't be sustained with less than 16.5% O2. Miners used similar lamps when canaries got to be too expensive (just kidding, they needed a light that wouldn't cause an explosion). Simply put, if a candle won't burn, you can't breathe the air and expect to live... but remember that a flame is consuming oxygen as it burns.
Carbon Monoxide (CO)CO is formed by the incomplete burning of organic material. CO is flammable (in the range from 12.5-74% by volume) as well as toxic (it binds to the part of your blood that normally carries oxygen, preventing your cells from getting sufficient oxygen). It is a colorless, odorless gas that will cause headaches at about 1-1.5% by volume, and death at about 4% after 30 minute's exposure. This is a nasty chemical that used to be piped into houses as “man-made gas” or “coal gas” for lighting and cooking, and was replaced by “natural gas” (methane). There are tons of battery operated CO detectors on the market, most of them are less than $30. Get one.
Carbon Dioxide (CO2)CO2 makes up about 5% of what you exhale with each breath. It can also be formed by fire, decomposing organic material, dry ice, and leaks in food service soda machines. CO2 is heavier than air and will settle into low spaces, like basements and cellars, and displace the oxygen that was there. CO2 will cause unconsciousness and death at 10% by volume, and intoxication at 5%. The regulatory limit for exposure is 0.5%. CO2 detectors are available, but they're hard to find (search engines assume that you're an idiot and meant to type CO so they don't give proper results). They're also not cheap, running around $100-125. For local suppliers, ask around at bar and restaurant suppliers or greenhouses.
This is a wide category that covers everything from hydrogen (H) to volatile organic compounds (VOC). Any gas or vapor that will burn in air is a flammable or combustible gas, and is one of the main things that commercial gas monitors check for. Something as simple as an idiot getting too liberal with a spray can of penetrating oil can ruin your whole day (been there, seen that) if there is an ignition source present. If you're going to be working around or expect to see combustible gasses, get a monitor/detector. Broken gas lines after an earthquake or tornado are common, but natural gas and liquid petroleum, both colorless, odorless gasses, have an additive that stinks (methyl mercaptan) just to make them easier to notice.
DustNormal household dust is an annoyance, made up mostly of dirt, dead skin cells, and minerals that are not a hazard. If you live in an older building with the potential of having asbestos insulation, dust caused by a natural disaster could be a long-term health hazard. Unfortunately, there is no simple test for asbestos, so if in doubt, get out the respirator. Wood dust, concrete dust, volcanic ash, and silica (fine sand) dust all have health risks similar to asbestos and are visible in the air before they reach dangerous concentrations.
Other dusts can be an explosion hazard. Any vegetable-based dust will burn rapidly. If suspended in the air and ignited, the flame will spread (propagate) fast enough to be classified as an explosion. Visual testing is the easiest: if you can see it in the air it is too much. At your own risk, toss a handful of flour into the flames of a campfire some time for a demonstration or do a search on “Cremora pots” for some interesting videos. We're not responsible for your lack of eyebrows, arm hair, or any other injury if you try these.
Chemical Warfare AgentsThere are people on this planet who have, and will use, chemical warfare. Terrorists have been known to use some of the simpler nerve agents, usually in subways or other enclosed spaces. Without the proper testing gear (which is normally only found in military units and is in the “if you have to ask you can't afford it” price range), the only method I can suggest is the miner's canary. Birds breathe differently than mammals, and are more susceptible to anything in the air. Canaries, chickens, geese, or even sky rats (commonly called pigeons) can be used as a warning method. If you're sitting inside watching the birds through the window and they start to fall over, it's time to take some action. Caged sky rats placed within sight can give a few seconds or minutes of warning, if this is a potential threat you want to prepare for.
If there are any questions or you would like me to expand on any of these, feel free to leave a comment here or on ourFaceBook page. I will try to answer as well as I can and I enjoy getting the feedback.