We live at sea level and regularly take our dog up to 10000 feet and never had problems. We as humans also have to acclimate, so we spend first night at about 8,000 feet and take it easy the first few days. More serious problems with altitude is the rough, rocky trails that can eat up paws and heat. Be sure to have doggie boots. Our dog has back fur that really absorbs heat. The sun is very intense at high altitude. We make a point of only going on trails that have frequent access to streams and lakes so our dog can swim a bit to cool off. Also, we let our dog drink any water that is available- our dog has never gotten sick from drinking scum water. Most mountain stream water is fine for dogs.