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For a science experiment for school, I have set up 15 Winogradsky columns, and will be looking at the microbial growth inside the columns over a period of 25 days.

I have read that, due to gas produced by the microbes, it is good to leave the cap loosely closed to prevent an explosion from gas buildup. However, I have also read that sulfate-reducing bacteria that grow in the column can produce toxic hydrogen sulfide gas. Wouldn’t the hydrogen sulfide also escape in small amounts and pose a health risk? What would be the correct protocol to follow here?

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Hydrogen sulfide can be toxic at high concentrations, however, the amount that your columns will make is likely very low, and will almost certainly degrade/oxidize into something safer before it hurts anyone (who isn't a small organism in the column!). It will noticeably smell bad at levels well below what is dangerous. So maybe just make sure that the columns are in a ventilated space.

Remember, what is happening in the column is basically just rotting organic matter, like leaving a bunch of food waste in a highly compacted, sealed environment. It is sort of intrinsically gross, but it isn't any more dangerous to your health than forgetting to take out the garbage for a month (actually, it's probably a lot less dangerous than that!).

For example, this guide says the following (after constructing columns with different inputs such as e.g. eggs):

You may have seen worms, snails, shrimp or other small organisms in the water, but probably not many (if any) in the bottle with the egg yolk, because hydrogen sulfide is toxic to most organisms!

So, depending on what you put in there, a very H2S-productive medium is likely to create enough H2S to kill tiny animals that are trapped inside the column. But apparently not enough to hurt large animals like you, as long as you don't huff the column.

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    $\begingroup$ You will smell H2S at well less than 1 ppm . Long term exposure problem threshold is about 10 ppm . I smell it when i stir up the bottom of my pond or aquariums. I have never heard of a toxicity problem with the levels of H2S produced in a natural environment . $\endgroup$ – blacksmith37 Nov 14 '20 at 22:08

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