This morning a workman came by to my newly rented property to fix a few things, among which a slightly blocked bathtub drain.

He let some mild water run continuously and he poured down a chemical, and it wasn't before I saw a huge vapor coming out and an intense boiling-like sound, I realized something was wrong. Basically he let down an entire bottle of >95% sulfuric acid down the drain just like that, without even wearing safety goggles or gloves. A pungent odor emanated through the whole flat and I had to open every window (he hadn't advised to open them beforehand).

Now, I don't have much experience with chemicals of this sort, so here are my questions:

  • Did I inhale toxic fumes?
  • What should I do to make the bathroom/shower usable again? I noticed that the mirror and several water tiles that are not in the close proximity of the drain but are on the way to the nearest window opening, show condensation streaks, such as those left by drops of water that run down. They were definitely not there before. Is that condensed sulfuric acid vapor? Do I need to clean with a sodium bicarbonate solution?
  • Needless to say, I also had towels, toothbrushes, and other items in the room, do I need to take any precaution before using them (cleaning, rinsing, etc?).

In short, how do I clean up this mess?


If the workman did use concentrated sulphuric acid (which is not the best choice for drain cleaning) then the best solution if to flush the drain with large quantities of water and wash the surfaces with water.

Sulphuric acid is not toxic but is very corrosive when concentrated. But it is highly water soluble and not harmful when it is dilute enough. So dilution with water will solve whatever problems it seems to cause. If it got splashed around, wash everything thoroughly and there will be no further problem. If you use enough water then it won't even be necessary to neutralise the result as any remaining acid will be swept through the drain and highly diluted by the flow of water.


He wasn’t wrong to be using Sulfuric acid, he just didn’t know what he was doing. Sulfuric acid is sold for cleaning pipes, but you won’t find it easily because it is one of the most dangerous ways to clear a drain.

The boiling-like sound was the acid reacting with compounds in the drain (hair, grease, soap, certain metals, etc.). During the reaction various gases would have been released causing the mixture to bubble and fizz.

There is likely a cocktail of chemicals in the clogged drain so it can be difficult to say exactly what gases will be produced, but the important ones are Hydrogen, Sulfur dioxide, and possibly some Hydrogen sulfide. Hydrogen sulfide is very poisonous. It smells like rotten eggs, but Sulfur dioxide has a similar smell. Hydrogen sulfide in low concentrations isn’t instantly deadly, but it can kill you slowly. In high concentrations one or two breaths of Hydrogen sulfide can be deadly. It is a popular chemical for committing suicide. But that will likely not be the main gas produced. The main point here is that forced air ventilation (a fan) should be setup to blow fumes out a window BEFORE you start using it.

Concentrated Sulfuric acid also reacts with water and releases a lot of heat (this is a whole separate dangerous property aside from it being a strong acid). The mixture in the drain can easily become hot enough to boil the water and when contained in a drain pipe it can erupt a geyser of hot water and acid straight out of the drain. Acid burns are unpleasant. Hot acid burns will send you to the hospital for skin grafts. What can take hours to react with acid at room temperature can happen in just a few seconds with hot acid.

But, yes, Sulfuric acid is used to clear drains, but it is an older method used by people who know how to handle it. Your plumber was clearly not one of these people. Sulfuric acid will instantly strip the nickel finish off the drain ring. Nickel is probably the most common finish on modern bathroom hardware. After contact with Sulfuric acid the fixtures will be black, gray, or yellow gold if it strips down to the brass plating.

Once the Sulfuric acid is drained away and the house airs out there will be no lingering vapors or chemicals to worry about. It’s nasty stuff but it doesn’t linger. If any Sulfuric acid splashed on fabrics or paper you will find the material burned away looking exactly like a cigarette burn. The end result of fire and Sulfuric acid is the same — black carbon.


I doubt that the workman was a professional.

Sulfuric acid has a high boiling point (338 °C) much higher than water so I don't think it would evaporate back out of the drain unless it created a reaction hot enough in the drain for it to sublime.

Without knowing what was in the drain to begin there could be many chemicals that could have been produced by pouring the h2so4 down the drain many of which could evaporate back out the drain, including compounds with higher boiling points than water can leave with the water because there attracted to and soluble in the steam, because this occurs some of the acid most likely evaporated back out of the drain.

Tap water will neutralize the acid but if it was 95 % which is extremely strong it would take a lot. Taking into account the acid would have been neutralized slightly by the gunk in the drain a few buckets of a weak bicarbonate solution should be OK. Don't make a strong bicarbonate solution or it'll foam up. It is not good practice to neutralize a strong acid with a strong base or other way around. Sodium carbonate is classes as a weak base and sulfuric acid is a strong acid so if you use a few buckets of a weak bicarbonate solution it should work.

As for cleaning the residues, if it really smells chemically bad like your saying I'd get a professional to clean it up holding the guy that did it responsibly. Don't mess with chemicals if you don't know what they are you might get sick. You may have breathed in toxic fumes it sounds like it. If you feel sick or are worried call the hospital. I'd definitely change toothbrush and towels and anything that goes in or on your body all of which should have been removed from the room prior to using a chemical that strong in the household.

Consider this, the sulfuric acid for car batteries is about 45 % and even that would not be practical to pour down a drain especially if there's metal components or metal pipes. If there is metal involved I'd say a lot of hydrogen would've been created. Hydrogen sulfide may have been produced which has an extremely pungent smell.

  • 1
    $\begingroup$ Thanks for your reply. Now it doesn't smell that bad, after I left for about 10 hours leaving all the windows open. This is the stuff that he used: (meltmsds)[markvitow.com/external/commerce/1/productfiles/safety/…. As for the water that would neutralize the acid, how much would it take? And how much and how strong should the bicarbonate solution be? I'm a bit worried that calling a professional cleaner the odds of any professional cleaner not knowing what's going on and using a bleach-based cleaner are quite high and that would make the situation worse. $\endgroup$ – A L Oct 23 '15 at 6:07
  • $\begingroup$ I don't know exactly how much tap water it would take but a few buckets . Bicarbonate will work better. I think if you used 1/4 cup of bicarbonate in a 9 L bucket of water a few times it should be fine. $\endgroup$ – Technetium Oct 23 '15 at 6:54
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    $\begingroup$ I was just reading your link. That guy was an idiot . That chemical is not for use in a home kitchen or bathroom that's an industrial drain cleaner I'd sue him for endangering your life. Have a read of the safety precautions or more importantly the conditions to avoid that are stated in the msds. $\endgroup$ – Technetium Oct 23 '15 at 7:00
  • $\begingroup$ Thank you for your response. Suing sounds like too much effort, but I will at least I will report it to his company and my landlord who arranged the work. I just can't believe how he could turn such a simple plumbing job into a chemical mess. $\endgroup$ – A L Oct 23 '15 at 8:37
  • $\begingroup$ He's an idiot , not the first , definitely won't be the last! It's definitely overkill for a bathroom drain! And as I said read the bit in the msds where it explains how and where not to use the product. If it really is 95% H2SO4 it may have damaged the drain system... $\endgroup$ – Technetium Oct 23 '15 at 8:59

Sulfuric acid seems like an odd choice to clean a drain. Typically drain cleaners are strong alkali mixtures.

Concentrated sulfuric acid ionizes on contact with water with a very exothermic reaction. It could easily boil or splatter up from the drain.

Rinsing with water would be sufficient to clean up any mess. To be on the safe side, I'd wash linens in bathroom before using again.

  • $\begingroup$ This is what he used: link $\endgroup$ – A L Oct 23 '15 at 5:57
  • $\begingroup$ I believe you. Just seems odd to me. The worry would be droplets of sulfuric acid which might have gotten blown out of the drain. The droplets of sulfuric acid won't evaporate. $\endgroup$ – MaxW Oct 23 '15 at 7:23
  • $\begingroup$ What does that imply? How should I clean up those droplets? Thank you. $\endgroup$ – A L Oct 23 '15 at 8:34
  • $\begingroup$ Rinse tub/shower with water. $\endgroup$ – MaxW Oct 23 '15 at 13:10
  • $\begingroup$ Water will only dilute the sulfuric acid, not neutralise it. Also the trap may also contain residual acid, so make sure plenty of litres / gallons of water go through it. Avoid bleaching products until this occurs as otherwise chlorine can be released from the NaClO $\endgroup$ – Beerhunter Oct 23 '15 at 18:25

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