Toxic levels of baking soda What is the amount of baking soda that would be considered toxic for skin? I ask because there is a lot of misinformation online about using baking soda as a "natural" underarm deodorant, and how it is or is not harmful to use as such. Please clarify.

Thank you for your time in advance.
Baking soda is sodium hydrogencarbonate, $\ce{NaHCO3}$, also known as sodium bicarbonate.

The only toxicological data in the GESTIS database on hazardous substances
 is a LD50 of 4220 mg/kg for the oral uptake by rats. There are no data for dermal uptake. 

In water, $\ce{NaHCO3}$ is weakly alkaline. I do not know whether it might cause skin irritations, particularly in more sensitive regions, such as armpits. 

In occupational safety, I'd recommend to avoid skin contact and use personal protection equipment when handling it.