Contact repellents repel insects as they touch the repellent-coated skin. They are usually slightly volatile, which encourages insects to land on the skin and make contact with the repellent. They are also called gustatory repellents.
Vapor repellents contain chemicals that release specific odors to repel insects before coming into contact with the skin. Some products use odors that insects naturally avoid, while others use strong odors to drown out human scents.
Electronic insect repellents use either ultrasound or electromagnetic waves to repel insects. They are usually plugged into power sockets and are commonly used in homes and large rooms. Ultrasound repellents work by emitting sounds in frequencies too high to be audible to humans, but can be heard by insects. They usually imitate sounds of natural enemies to drive insects away.
Electromagnetic repellents cause minor fluctuations in the current generated by household appliances. These fluctuations are not felt by humans but create a hostile environment for insects and other pests, forcing them out of the house.
Types according to application
Topical repellents are lotions or creams applied on the skin. Most types are made up of DEET and a variety of aromatic chemicals. Some use oil of lemon eucalyptus and another chemical called picaridin, although these types are not recommended for children under three years. They usually work within a limited range, so users may still see mosquitoes nearby when using the repellent.
Many varieties use mild or hypoallergenic ingredients to make them safe to use on babies or those with sensitive skin.
Clothing repellents are thin, light liquids sprayed onto clothing. The main ingredient is usually permethrin, a strong chemical that kills insects prior to contact. They are usually applied to shirt sleeves, cuffs, hats, and shoes. They can also be sprayed on camping gear and beddings, including sleeping bags and mosquito nets. They are not effective on skin, and stronger variants may even be harmful with direct contact.
Room sprays are stronger mixes that are sprayed within closed rooms. They last longer than other repellent types, with some brands lasting up to eight hours. Some are also formulated to be used as skin sprays as well. Many contain natural ingredients and come with floral scents to give the room a pleasing smell.