Thiourea/thiocarbamide is NOT a proven carcinogen

Hi all,

I have seen this rumour going around for some time now here on APUG, and I think it is time to put it to rest. I have seen or read of multiple people scaring away of using odourless thiourea/thiocarbamide based sepia toner (the latter "carbamide" is an old deprecated name for the same substance), because of it supposed carcinogenicity. Now I looked this up, and the situation seems to be quite different from the scary stories.

Thiourea is classified in the same league as the normal hydroquinone everyone uses in its developers in terms of carcinogenic risk.

It is actually classified in class/group 3 of the International Agency for Research on Cancer (IARC) classification for carcinogenicity. This is an authoritative and independent source, as the IARC is part of the World Health Organization (WHO), so I guess we can trust this.

Group 3 according to IARC:
Group 3: The agent is not classifiable as to its carcinogenicity to humans.

You can find thiourea's classification in this PDF document of IARC. Please note you MUST search for the substance with ONLY "thiourea" as the name to find the right data, as there are other substances with "thiourea" in part of their the name. Thiourea has CAS no. 000062-56-6.

*** This means the risks are probably limited, as current research does not state or support conclusively it is a harmful substance. ***

Actually, hydroquinone as one of the most common paper and printer developers, according to the Kodak 2009 D-76 datasheet, is classified in exactly the same category :confused: :

The International Agency for Research on Cancer (IARC) has classified hydroquinone in Group 3, i.e., "not classifiable" as a carcinogen.

Of course, you can also look up Hydroquinone in the IARC document, it has CAS no. 000123-31-9.

So both hydroquinone and thiourea, definitely are not ranking - at least based on current data and insights - as proven human carcinogens, as that would be Group 1, nor as highly suspicious (Group 2A: probably), or possible (2B) carcinogen.

Still, a bit of precaution and easy avoidance of contact by proper usage of tongs or gloves, can't hurt either, but I see no reason not to use it with regular common sense.

Marco

Actually, to be a bit more precise, "not classifiable" in very strict terms means that more research is needed to either prove or disapprove it is a carcinogen and thus put it in a certain "class", but in reality, based on for example peoples exposure to hydroquinone, I think any effects may be so small so as to be extremely hard to prove. E.g. if effects are very small, like say with the stories surrounding high voltage power lines and leukaemia, it may require an almost impossibly big sample number of people to prove an effect with scientific 95 or 99% confidence.

good job. thanks.

By the way, to make the "non-classifiable" a bit more clear: it is important to note that "non-classifiable" is not equivalent to "proven not-carcinogenic to humans". It maybe that there simply is not sufficient research data (or non at all) to classify it. However, in case some substance has been researched and it would be highly carcinogenic, it is unlikely to fall in group 3, as even a quick study would most likely provide conclusive evidence it is carcinogenic, unless the research had been done very bad.

Actually, looking at the classifications of the more than 900 substances IARC evaluated, only 1 substance was classified as "Group 4 - Probably not carcinogenic to humans", most others fall in group 3...

It clearly also takes very good research and evidence to have a substance fall in this last category, not surprisingly considering the topic...

See the explanation by IARC here for the different groups, and the one for group 3 that I reproduced below.

"Group 3: The agent is not classifiable as to its carcinogenicity to humans.

This category is used most commonly for agents for which the evidence of carcinogenicity is inadequate in humans and inadequate or limited in experimental animals.

Exceptionally, agents for which the evidence of carcinogenicity is inadequate in humans but sufficient in experimental animals may be placed in this category when there is strong evidence that the mechanism of carcinogenicity in experimental animals does not operate in humans.

Agents that do not fall into any other group are also placed in this category.

An evaluation in Group 3 is not a determination of non-carcinogenicity or overall safety. It often means that further research is needed, especially when exposures are widespread or the cancer data are consistent with differing interpretations."