Old, sick, or injured animals may have poor blood pressure or weak hearts, which may delay the effects of CO, causing them to experience distress prior to unconsciousness. Animals under the age of four months may not have the lung capacity to inhale enough CO to be effective. In pregnant animals, it is likely the mother will die from exposure to CO before the unborn puppies/kittens. Consequently, it is possible the puppies/kittens may die as a result of the mother’s death (by suffocating to death) rather than from exposure to CO. The HSUS condemns using CO for animals meeting the above criteria.