Hydroquinone is used in skin-whitening creams at about 2-3%... It has nothing to do with this mysterious "residual hydroquinone", it's just that some DIY people seem to have found this oddly named product and want to make their own HQ skin creams, as some ready-made HQ skin creams are quite expensive, apparently.

My guess is that this "residual" thing is just some mysterious typo on Photoformulary product listing and nothing more... Photoformulary is well known for a bit... clunky marketing and website .

It is interesting how some people are ready to apply a "developer" to their skin directly for long contact times and repeatably every day, while, at the same time, there are people who are afraid of even trying to develop photographs because they are so scared of these chemicals.

I use this skin cream as an example to show people that they don't need to be that afraid; just be normally careful. If you get one drop of a developer to your hands, which happens every now and then when developing papers, and wash it out, it is still less than one of the millionth of the exposure thousands of people get on purpose without evident problems.