My highrise has a restaurant on the ground floor, and recently two burners were left on overnight. Around 6 am, the fire department was called; supposedly gas could be smelled on the 10th floor near the elevator shafts. Now fearful residents are spreading rumors about how dangerous this situation was and how we escaped disaster by the skin of our teeth.
My understanding is that gas can be smelled at a tiny fraction of the concentration where it becomes explosive. But what are the actual numbers for that difference?
Natural gas is odorless after being processed, thus utilities will generally dose it with mercaptan which gives it that distinct sulfurous smell. Natural gas is typically only explosive in the range 5% to 15% in air, however local conditions and concentration effects can be complex so it is hazardous no matter the concentration. Additionally, it is an increased fire hazard due to ease of ignition.
There is an entire ASTM testing standard on just this question:
ASTM D 6273.98 Standard Test Method for Natural Gas Odor Intensity
Mercaptan is added to natural gas in a concentration typically between 0.5% and 1%, at least locally here. There may be variance in other jurisdictions. Because the sense of smell is variable among the population, the threshold for straight methyl-mercaptan detection is commonly given as around 0.0021 ppm. This relates to some confidence interval of detection amongst a population group - not sure on the details, I’m afraid.
Assuming it is dosed at 1% into the gas stream, all other things being equal, the *threshold *for detection of the gas/mercaptan mixture would be around .000021% then. Of course the mixture of the natural gas and the surrounding air is not going to be homogeneous in the normal setting, there will be local concentrations, and the gas naturally vents upwards and disperses easily, so the practical outcome is going to be quite variable based on the person doing the smelling and the specific room, area and source of gas.
in your situation, the gas would have made its way into the elevator shafts as it is lighter than air and the shafts offer a convenient way for gas to traverse between floors, and then probably concentrated itself it the 10th floor due to some natural obstruction.
That’s the way i calculate it, yes. However, typical caveats - that number is the lowest threshold at which it can possibly be detected by the normal human nose. Real life situations would mean a significantly greater % is required to actually detect it, I think - based on the persons individual sensitivity, their capacity to recognize the smell for what it is, the local conditions in which they are attempting to smell it, any background odors that might be confusing the issue?
I know nothing about the physical aspects or psychology of the sense of smell so that actual number could be far greater outside of the lab.
Plus, if the gas was smelt at 6 am that could mean that mean that it didn’t reach the required concentration until then, or it could mean that it had for some hours been very much above the required concentration, but since nobody was wandering around the lift doors at 3 am or 4 am nobody smelt it.
Gas leaks inside buildings, if unresolved, can result in explosive concentrations, and can result in explosions. Probably an explosion was not imminent in this case, but there is no way of knowing. However high the concentration on the 10th floor was, the concentration in the kitchen of the ground-floor restaurant may have been much higher; we know nothing about how well-ventilated that space was.
In most cases, for methane, I’d worry about explosion first. Methane doesn’t directly interfere with breathing, so the only problem is if there’s so much of it that there’s not enough oxygen in what you’re breathing. That would require a really high concentration (though if you were in 90% methane, it’s true you wouldn’t need to worry about an explosion, as there’s not enough oxygen to burn the methane, either).
Also, you’d start noticing as the concentration of methane went up: you’d start feeling short of breath (that’s assuming this is unscented methane. You’d very very definitely notice if there was enough gas-line methane to even make you slightly short of breath).
Finally, since methane is lighter than oxygen/nitrogen, it floats. So a big leak will tend to just find its way out of any building. It’s hard to just walk into a big cloud of methane. (Gases that are heavier than air are scary, because they’ll pool in a basement or something, and when someone walks into the basement, baff, they fall unconscious. Then someone else sees them fall and runs in to help, baff two. )
In my younger-and-dumber days, I lived in a rental home with some friends. One day, we noticed a slight gas smell in the kitchen. We opened the downstairs windows, ran the ceiling fan in the adjacent living room, and so forth. We continued to use the gas stove.
Turned out that there was a gas leak in the line leading to the stove. We didn’t get it conclusively diagnosed and repaired, though, for a few days. No one in the home smoked, and I guess our efforts to air out the kitchen and living room made enough a difference so that the stove didn’t ignite any free gas that happened to be in the air.