Borax (sodium borate) is an alkaline mineral salt.
It's a naturally occurring element but even things from nature can be harmful (like asbestos or mercury). Historically, it was a popular and effective cleaning product because it disinfects, fights mold and mildew and whitens. Our grandmothers always had a box on hand.
By 2000, borax appeared in many DIY cleaning recipes as an eco-friendlier option to petroleum-based ingredients in conventional cleaning products. (A few of my original green cleaning recipes did contain borax.) But by 2010, I changed most of my green cleaning recipes, like borax-free liquid laundry soap, to reflect the European Union calling the safety of borax into question. The E.U. classified boric acid and borax as reproductive toxins. Yet they said consumer exposure to low doses of borates in cleaning products falls below levels of concern.
And ever since then, the safety of borax as an ingredient in green cleaning recipes has been confusing. (I have an open box collecting dust.)
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Adria Vasil of Ecoholic tries to explain:
"A few years ago, the European Union said boric acid/borax were reproductive toxins at high levels but not endocrine/hormone disruptors (which can trigger problems at quite low levels). At that point, they issued another report assessing the actual risk to consumers and they said levels used in detergents/soaps/cleaning products were safe. Then things shifted. They ended up putting boric acid on the list of potential hormone disruptors. I think it's important to clarify that the list basically prioritizes substances for further research for endocrine disrupting effects. The list contains some seriously troublesome chems, like known toxin PCB, as well as BPA, on which hundreds of studies have found hormone disrupting effects. Boric acid doesn't have the same lengthy rap sheet in terms of hormone disruption. So a lot of people are confused. Should we should keep using it with abandon? Should we shelve it for good? Well, I think we should certainly pause and wait for more research. It's also unclear why the EU only put boric acid on the list and not borax, when they've lumped the two together before. However, they are chemically different. So we'll assume borax was intentionally left off the list and is of less concern to consumers then boric acid, at these quantities."
In 2016, Health Canada did a screening assessment of boric acid. "The Government of Canada is proposing that boric acid, its salts and its precursors may be considered harmful to human health at current levels of exposure. Natural sources of boric acid in food (for example, fruits and vegetables) are not considered to be a health risk."