It depends on your set-up. In the winter we let our hens into the polytunnel so they can dust bathe in dry soil, but for the rest of the year they are not allowed in. So, if it's raining then they might not be able to find anywhere dry to bathe. However, ours have now made themselves a batheing area in a dirt floored leanto. We did used to leave a tray of wood ashes under cover for them, and they loved it, and sometimes now we add ashes to their favourite spot. We never had respiratory problems when our hens did use ashes alot, but it could well be a risk factor. I suppose you could add diatomaceous earth to their batheing hollows, or to soil in a box, and that would help against passengers, which is what wood ashes do. I used to be doubtful about the use of DE but now I like it.