The answer as to whether or not dolphins can sneeze is not as cut and dry as giving a simple yes or no answer. Unlike humans and most land mammals, dolphins (and whales and porpoises) inhale and exhale oxygen differently. In fact, humans and most land mammals are unconscious breathers meaning we do not have to think about breathing to survive.
When a land mammal sneezes, it is an unconscious and reflexive response. The involuntary sneezing that land mammals experience is their body’s way of getting rid of possible irritants or toxins that the animal accidentally breathed in a while inhaling oxygen.
Dolphins, whales, and porpoises, on the other hand, are always conscious of their breathing and must be consistently aware of their environment because they live and the ocean and could drown if they accidentally ingest water into their lungs.
This constant awareness of their breathing and environment means that dolphins are always aware of their need for oxygen and can consciously choose when they inhale and exhale as well as giving them the ability to control their bodies need to remove irritants or toxins from their respiratory system through the blowhole, removing the possibility of accidentally sneezing, which is extremely important when underwater as it could accidentally open the dolphin’s nostrils and cause water to enter.
While dolphins do not sneeze in the way a land animal does, these marine mammals can voluntarily cough or exhale oxygen at a swift pace when they are sick or suffering from a respiratory infection, which allows the dolphin to remove debris or germs that could cause respiratory infection or block the dolphin’s air passage.
In addition to the fact that dolphins are conscious breathers and have consistent control over their ability to breathe, these amazing marine mammals also possess another big difference over land animals.
Unlike most land mammals, dolphins do not breathe through their mouth. Instead, the dolphin’s air passage and lung passage are completely separated, allowing the dolphin to consume food without accidentally inhaling water into its lungs. From an anatomical standpoint, the food passage is located in the dolphin’s mouth, while the air passage is located on the top of its head through its blowhole(s).
Having their blowhole on the top of the head also allows dolphins to easily take in oxygen without having to lift their head completely out of the water. After exhaling or coughing, the dolphin can quickly inhale more oxygen before diving back under the water’s surface. When diving underwater, the sphincter muscles surrounding the blowhole contract preventing water from getting into the blowhole and lungs.
So, to sum up, the answer as to whether or not dolphins sneeze, dolphins do not involuntarily sneeze the way land mammals do. Instead, dolphins can voluntarily cough and exhale oxygen quickly, which allows them to clear their air passage and lungs of debris, germs, and other irritants that can cause them respiratory or breathing issues.
As with the dolphin species, all cetaceans (which include all whale and porpoise species) breathe in the same way and control their sneezing or coughing. A very similar answer can also be found on this post about whether or not whales have the ability to sneeze: Can whales sneeze?