Walnut dust can be toxic to some people and the shavings are toxic to some animals. There is some kind of "poison" in walnut dust and you should always use a mask. I have been using and cutting walnut for 40 years and always sand outside or have the fan blowing and a mask on. My father never seemed to mind it and would stand there with the dust from his slack belt sander all over the place and never seem to be bothered. I couldn't even come in the shop.