Alrena, who is inside the house, says that she can make a gas mask if you can get her some dwellberries. You can pick up some dwellberries inside the McGrubor's Wood enclave, by entering through the railings on the northern boundary, but beware of the level 33 guard dogs within. Give her the berries, and she'll make you a mask. If you lose the mask, there will be one in the wardrobe. Make sure you get a picture of Elena while you're inside the house.