Burning wood can leave creosote deposits that are typically responsible for fireplace odors. During the warmer months or on days with elevated humidity the smells are likely to increase. Preventions are annual cleanings and inspections. Some fireplaces still require further agents to combat smells such as commercial chimney deodorant, baking soda, or even kitty litter. You may also need to have a top-mounted damper installed to stop or slow air from moving around on the inside of the fireplace.