Cats don't normally allow dirt to build up on their skin or fur, and if they do, it's often a sign that something is wrong. This is true for flea dirt, which is an indication that fleas have infested your cat so heavily that he isn't able to keep up with amount of filth they leave behind.
Flea dirt is actually excrement deposited on your cat by fleas. It contains digested blood, giving it a red or rusty tinge. It looks like small red or brown flakes, similar to dandruff, except in color. One way to determine whether the flakes are flea dirt or some other substance is to wipe it with a damp white cloth or tissue. If it dissolves and leaves a dark red streak that looks like dried blood, it is flea dirt. If it does not dissolve or leaves a plain brown mark like dirt, it's probably something else. If there is flea dirt on your cat or on his bedding, it means that he is infested with fleas. Since most cats are fastidious about grooming themselves, it is unusual to find flea dirt unless the infestation is severe.
Bathing is optional to remove flea dirt. It is important, however, not to just clean away the dirt, but to implement a program to rid your cat of fleas. A medicated bath can provide relief to the irritation caused by flea bites and kill adult fleas. It also prevents matting and additional skin problems in severe cases. It can wash away other topical flea treatments, however, and dry your cat's skin if done too often. If you choose to bathe your cat, use only products labeled for cats since shampoos with medicine or insecticide designed for dogs can make your cat sick.
If you don't want to bathe your cat, or if the stress it might create is not worth the benefits, there are some other options for removing flea dirt. Use a brush with soft bristles to gently sweep the dirt away from the skin and off of your cat's fur. Be careful not to brush too vigorously, which might irritate your cat's sensitive skin. Or use a damp cloth or pet wipes to spot-clean areas that are especially dirty. You also can allow your cat to groom it away himself.
Dips and baths may kill adult fleas and provide temporary relief, but they don't break the pest's life cycle or offer continuous protection against infestations. A complete treatment protocol is necessary to get rid of fleas and prevent additional flea dirt. This should include a monthly application of a residual insecticide that contains insect growth regulators to interrupt the flea's reproductive cycle on your cat, and household sprays and cleaning to remove fleas from your cat's bedding and living space.