I work at a food manufacturing plant and we use Tetra Pak packaging for our liquid foods. There is a layer of aluminum foil within the packaging material that we need to dissolve out of the package in order to examine the sealing of the inner plastic layer. We will be using 30% HCl for the reaction. We have a ductless fume hood with an acid gas filter for this process. I am concerned that the hydrogen gas released in the reaction could be an issue in the room as it is unlikely to be filtered out by the hood. We will be doing, at most, 16 packages a day (all at the same time) and the reaction would likely last for 45-90 minutes. Also, it would not be the entire package being dissolved but only areas near the seal being examined. Is there a reason to be concerned about the hydrogen gas released in this reaction when using a ductless hood? The lab is not particularly small and reasonably ventilated. If necessary we could locate the hood to an area outside the lab with better ventilation. Any input is greatly appreciated and I can provide more details if needed.

  • $\begingroup$ You seem to be afraid of something. What ? Hydrogen is a light gas. It is not considered as toxic. Its density is rather low. So it will leave the lower part of the hood. It burns or explode in a reaction with oxygen. Is it what you are afraid of ? $\endgroup$
    – Maurice
    Commented May 17, 2023 at 8:56
  • 1
    $\begingroup$ My concern may be unjustified. I am setting up this procedure and may have some nervousness that I’m not overlooking anything. My concern would be outside of the hood and if the hydrogen gas could be enough to combust. Our lab has a lot of other equipment that may be running such as an autoclave, water distiller, solid analyzer (basically microwave), and drying oven. My background is in microbiology so I feel slightly out of my comfort zone. $\endgroup$
    – Andrew
    Commented May 17, 2023 at 14:24

1 Answer 1


Hydrogen safety states, "hydrogen has no rating for innate hazard for reactivity or toxicity," so the primary concern is flammability. However, without measurement, it is impossible to give a definite answer.

  1. Measure the mass of the aluminum in the foil. Either weigh the foil directly, or, if attached to a plastic layer, measure the mass of the whole before and after aluminum is removed to find the difference, i.e., mass of Al.

  2. Calculate how much hydrogen that releases over 45 minutes, the worst case scenario. Given ventilation in the hood (calculate the rate of air exchange, either from the hood's specs, or using an anemometer), could the volume of hydrogen compared to the volume of air moved over that period of time approach the flammability limit of 4% hydrogen:air in the hood?

  3. Calculate how much hydrogen could be released from 16 tests over a work-day (8 hours?). Given ventilation in the room (information that should be available from physical plant staff, since there would have been design specs for HVAC air exchange in the facility), could the volume of hydrogen compared to rate of air exchange in the room over that period of time approach the flammability limit of 4% hydrogen:air?

[Now, for my totally unjustified assumption: you're likely dealing with milligrams of aluminum, releasing a few milliliters of $\ce{H2}$ for each test. It seems unlikely, if you could capture all the gas from one test in a balloon, that you'd hear a pop! if you lit it.]

  • $\begingroup$ Your assumption is likely correct. I will plan to do some measurements to figure out the amount of foil that will be dissolved and if that can result in dangerous levels of hydrogen in the air at any point during the process. We would ideally be doing this with all the test packages at one time so the concern would be high levels at the peak time of the reaction. Thank you for the information. $\endgroup$
    – Andrew
    Commented May 17, 2023 at 14:10
  • $\begingroup$ Then, as you state, just multiply the amount of hydrogen by 16. BTW, if the amount of H2 is an issue, it could be "flared off" by leaving a small pilot light (e.g., an alcohol lamp) directly above the reaction vessel, so as to burn it off before it accumulates. A hot platinum-metal (Pt, Ir, Pd, etc., but not Os) wire could serve the same purpose. Again, it would seem unlikely much H2 would be released, unless the Tetra Pak is quite large. $\endgroup$ Commented May 17, 2023 at 15:11

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