Another of my "hot buttons".
If rust stain remover is safe for fabrics it is usually hydrofluoric acid (also used for etching glass) or occasionally oxalic acid (also used for bleaching wood). They are dangerous because they will not burn your skin, but will migrate through the skin into the tissues and attack the nerves in the body. It is very difficult to treat if this happens, and you will know by the incredible pain, but often with no redness or irritation showing on the skin.
I strongly suggest using rubber gloves with any acid. I am very afraid of hydrofluoric acid (household "Rust Stain Remover", often in a brown bottle in the cleaning section of the store), and somewhat so with oxalic acid. I like oxalic acid for its general stain removing qualities, though, and keep a container of the crystals on the boat. But, because these do not burn when they touch your skin, it's easy to believe that it is safe enough to use without rubber gloves. It isn't.
I first read of the danger of hydrofluoric acid in DISCOVER
Magazine. Here's a link to the article: An Invisible Fire
Hydrochloric acid (also called Muriatic acid), and phosphoric acid, also remove rust and rust stains. They can burn the skin, are highly reactive and have a greater propensity for splashing up and into one's eyes.
The bottom line, of course, is to be cautious with any acid, and don't allow yourself to be lulled into not taking appropriate precautions.