Mosquito Repellent

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Mosquito Repellent

More than one-third of the U.S.population uses aDEET-based insect repellenteach year.

A mosquito repellent is a substance designed to keep away mosquitoes, thereby preventing them from biting humans and feeding on human blood. It typically contains an active ingredient that repels mosquitoes and secondary ingredients that, among other things, dilute the active ingredient to a desired concentration and help in releasing the active ingredient when needed. Mosquito repellents are available as creams, lotions, oils, and sticks, which are applied directly on the skin. They also come as aerosol and pump-spray products, which are used on the skin and to treat clothing. Mosquito repellents are different from insecticides in that the latter are used to kill insects.

Do they really work?

Various substances have been used to repel mosquitoes. These include smoke, tar, mud, and oils derived from plants, such as citronella, cedar, geranium, and peppermint.

Citronella oil, derived from the Southeast Asian grass citronella, was the first truly effective mosquito repellent. Citronella had been used for centuries for medicinal purposes. In 1901, it was discovered that citronella used as a hairdressing fragrance could repel mosquitoes. However, the oil evaporates quickly and, therefore, lasts only a short time. Citronella oil is also used as the active ingredient in candles or coils that are burned outdoors to produce smoke that repels mosquitoes. They have been found effective only if air movement is minimal.

Manufacturers of tablets containing garlic, brewer's yeast, or Vitamin B1 claim that their products repel mosquitoes. Some electronic devices that vibrate or emit high-frequency sounds also make such claims. Scientists have found that some lotions advertised as insect repellents do not contain any repellent. Instead, the mineral oil in these lotions serves as a film that prevent insects from penetrating the skin.

A government invention

During World War II (1939–45), American soldiers stationed in the Pacific carried insecticides in aerosol cans to protect themselves against mosquitoes and other insects that carry diseases, such as malaria. After the war, in 1946, the U.S. Army and the U.S. Department of Agriculture jointly developed the repellent ingredient DEET (N,N-diethyl-3-methylbenzamide, formerly known as N,N-diethyl-3-m-toluamide). DEET became available to the public in 1957. Since then, thousands of chemical products have been tested, but DEET remains the only active ingredient that delivers the effectiveness required of a useful mosquito repellent.

Repelling action

Mosquito repellents work by interfering with the insect's homing system. This homing system, located in the antennae, consists of chemical receptors. The chemical receptors are stimulated by lactic acid that naturally evaporates from the skin of warm-blooded animals. When a repellent ingredient, such as DEET, is applied to the skin, it evaporates, forming a barrier around the skin. The mosquito, therefore, is unable to "find" the person to bite.

Raw Materials

A mosquito repellent consists of active and inert, or inactive, ingredients. The active ingredient is mainly responsible for the product's usefulness, that is, it effectively discourages mosquito attacks on the treated human skin and/or clothing. The inert ingredients are not chemically active. They are added to the product to perform different functions. For example, fragrances make the product cosmetically acceptable.

DEET is the active ingredient in the most widely used insect repellents. According to the Environmental Protection Agency (EPA), more than one-third of the U.S. population uses a DEET-based insect repellent each year. More than two hundred million people worldwide use similar products.

TIMELY INVENTION

In 1942, U.S. Department of Agriculture researchers Lyle Goodhue and William Sullivan developed the first aerosol insecticide (a chemical substance used to kill insects). This invention proved very helpful to American soldiers stationed in the Pacific during World War II (1939–45), where they were constantly attacked by mosquitoes and other insects that carried diseases.

Other repellent ingredients include the chemical substance dimethlylphthalate and plant-based repellents, such as citronella oil, lavender, lemongrass oil, and peppermint oil. It has been found that mixtures of different repellent ingredients are more effective than any one alone. The active ingredients in mosquito repellents generally make up about 5 to 30 percent of the final product.

The inert ingredients used in a mosquito repellent depend on the form that the product will take. Mosquito repellents are sold as aerosols, pumps, creams, lotions, oils, and sticks. Those that are sold as creams or lotions are essentially skin creams, with a certain level of DEET added. They mainly consist of water, emollients (skin softeners), fatty alcohol, fragrance, and surfactants (substances that keep the liquid ingredients spread out). They act both as a skin moisturizer and a repellent. However, they are not as effective as aerosols or pumps because the active ingredient cannot evaporate easily.

Aerosols, the most common form for mosquito repellents, consist of a solvent, a propellant, and other ingredients. The solvent, propyl alcohol or ethanol, dilutes the active ingredient to a specific concentration. It also keeps all the materials mixed so the product will remain effective even after long storage. The propellant is a high-pressure gas that forces the repellent out of the container when the button on top of the can is pushed down. Propellants used include liquefied hydrocarbon gases, such as propane, butane, or isobutene, hydrofluorocarbons, and dimethyl ether. Fragrances and emollients are added for cosmetic appeal, as are substances to prevent rust and keep the product stable (maintain its chemical and physical characteristics).

The aerosol can is typically made of tin-plate steel, or steel that has been coated with tin to prevent any interaction between the steel and the repellent ingredients. The valve is a device that will open or close to control the movement of the repellent within the can when the actuator button is pushed down. The valve has three sections—the long plastic tube called a dip tube that extends from the bottom of the can to the top, the valve body, and the actuator button. When the actuator button is pushed down, the propellant pushes the mosquito repellent up the dip tube and out through a small opening that runs through the button. The liquid repellent comes out as a fine spray.

The Manufacturing Process

Since most mosquito repellents are marketed in the form of aerosols, the following steps describe the production of repellents contained in aerosols. The manufacturing process consists of two steps—the production of a large batch of the repellent formula and its packaging into aerosol cans. Other forms of mosquito repellents, such as creams and lotions, are made in the same manner. However, the filling process is not as complex.

Compounding

1 The raw materials are mixed in the compounding area. For an aerosol, the alcohol is pumped into a large stainless steel tank. Then, the DEET, fragrance, and emollients are manually poured in and allowed to mix. At this stage of the production, all the other ingredients are added, except the propellant. Since some materials are flammable, special precaution is taken to prevent explosion, such as using spark-proof electrical outlets and blast-proof walls.

2 When the batch is made, a sample is tested to make sure the product meets specific standards. The repellent is then pumped to the filling lines to make the finished product.

Filling

3 The filling line consists of a series of machines connected by a conveyor belt system. The machines assemble all the components (parts) to make the finished mosquito repellents. The first machine has a large hopper, or bin, containing empty cans. The cans are set in an upright position and fed onto the conveyor belt.

4 The cans are moved along the conveyor belt and cleaned with a jet of compressed air to remove any impurities. As the cans pass through a filling carousel, they are filled with a predetermined amount of repellent.

5 The next machine has a hopper that contains valves, which are sorted and aligned. The filled cans are topped with valves. A valve crimping machine attaches the valves to the cans. Depending on the filling method, the propellant is either injected through the valves under high pressure or injected into the cans before the valves are permanently attached.

BLOOD SUCKERS

Only female mosquitoes bite, using a needle-like sucking tube called the proboscis, to fill herself with blood from warm-blooded mammals and birds. Mosquitoes use the protein in blood to produce over two hundred eggs per blood meal. A female mosquito needs a blood meal each time she produces eggs.

6 After the cans are capped, they are immersed in a tank filled with hot water. The immersed cans are checked for escaping bubbles that would indicate a propellant leak. The high temperature of the water also raises the pressure inside the can. The increased pressure would cause any weak spots in the can to fail to operate. This quality-control step weeds out damaged cans.

7 After the waterbath, the cans are dried by high-pressure air jets. Other parts, such as the depressible button and the overcap, are added. Then, a label or printing is added.

8 High-speed production lines like the one just described can manufacture about two hundred or more canned repellents per minute. The finished aerosol cans are moved to the packaging area, where they are boxed, typically a dozen cans to a box. Lastly the boxes are shipped to the distributors.

Quality Control

Tests are performed at various points of production. This is to ensure that the finished products are consistent throughout the manufacturing process, will remain effective over a long period of time, and are safe to use.

Before the start of production, the incoming raw materials are checked against set specifications. The empty cans are checked for dents, rust, and other weaknesses.

During the different points along the filling line, samples of the repellent are tested for such characteristics as the level of active ingredient, pressure, spray rate, and spray pattern. The finished cans are also checked to see if they are dispensing the repellent properly. Long-term studies may also be done to establish that the cans do not show undue signs of chemical changes, such as rust.

The Future

In the last twenty years, scientists at the Agricultural Research Service of the U.S. Department of Agriculture (USDA) have tested about 16,000 new repellents as possible DEET alternatives. A repellent called piperdine may be a possible candidate. However, until studies of synthetic repellents are completed, DEET remains the most effective in repelling mosquitoes.

Both the USDA and the EPA believe that, when used as directed, mosquito repellents are safe for children. However, reported misuse of DEET-based repellents, especially among children, has prompted the EPA to impose labeling restrictions on products marketed for infants and children. The EPA considers as mislabeling any claim that a product is "child safe" or "for children." This includes pictures specifically targeted for children. In addition, manufacturers may not incorporate ingredients with food fragrances that may cause children to ingest the product.

aerosol:: A substance packaged under pressure and released as a spray.
DEET:: The active ingredient in the most widely used insect repellents.
emollient:: A substance that softens the skin.
patent:: To obtain from the government the right to make and sell an invention for a certain period of time.
malaria:: A serious disease spread by the bites of female mosquitoes. It is characterized by intense fever and may cause complications affecting the brain, blood, liver, and kidneys that can cause death.
propellant:: The high-pressure gas that forces the repellent out of the container when the button on top of the can is depressed.
solvent:: A substance in which another substance, usually a liquid, is dissolved.
surfactant:: A substance that keeps a liquid dispersed, or spread out, so that it does not pull together as droplets.