Insect repellents help prevent and control the outbreak of insect-borne diseases such as malaria, Lyme disease, Dengue fever, bubonic plague, and West Nile fever. Pest animals commonly serving as vectors for disease include the insects flea, fly, and mosquito; and the arachnid tick.
Common insect repellents include:
Synthetic repellents tend to be more effective and/or longer lasting than 'natural' repellents. However, some plant-based repellents are comparable to, or somewhat better than synthetics - depending on the formula. Essential oil repellents tend to being short-lived in their effectiveness due to their volatile nature.
A test of various insect repellents by an independent consumer organization found that repellents containing DEET or picaridin are more effective than repellents with 'natural' active ingredients. All the synthetics gave almost 100% repellency for the first 2 hours, where the natural repellent products were most effective for the first 30-60 minutes, and required reapplication to be effective over several hours. However, some products in the market like essential oil candle and natural herb mosquito coil can give protection to an entire room up to 8 hours.
For protection against mosquitos, the U.S. Centers for Disease Control issued a statement in May 2008 recommending equally DEET, picaridin, oil of lemon eucalyptus and IR3535 for skin. Permethrin is recommended for clothing, gear, or bed nets. In an earlier report, the CDC found oil of lemon eucalyptus to be more effective than other plant-based treatments, with a similar effectiveness to low concentrations of DEET. However, a 2006 published study found in both cage and field studies that a product containing 40% oil of lemon eucalyptus was just as effective as products containing high concentrations of DEET. Research has also found that neem oil is mosquito repellent for up to 12 hours. Citronella oil's mosquito repellency has also been verified by research, including effectiveness in repelling Aedes aegypti, but requires reapplication after 30-60 minutes.
Regardless of which repellent product used, it is recommended that the label is read before use and directions carefully followed. Usage instructions for repellents vary from country to country. Some insect repellents are not recommended for use on younger children.
In the DEET Reregistration Eligibility Decision (RED) the EPA reported 14 to 46 cases of potential DEET associated seizures, including 4 deaths. The EPA states: " ..it does appear that some cases are likely related to DEET toxicity," but observed that with 30% of the US population using DEET, the likely seizure rate is only about one per 100 million users.
The Pesticide Information Project of Cooperative Extension Offices of Cornell University states that, "Everglades National Park employees having extensive DEET exposure were more likely to have insomnia, mood disturbances and impaired cognitive function than were lesser exposed co-workers".
The U. S. Environmental Protection Agency states that citronella oil shows little or no toxicity and has been used as a topical insect repellent for 60 years. However, the EPA also states that citronella may irritate skin and cause dermatitis in certain individuals. Canadian regulatory authorities concern with citronella based repellents is primarily based on data-gaps in toxicology, not on incidents.
Some old studies suggested that the ingestion of large doses of thiamin could be effective as a oral insect repellent against mosquito bites. However, there is now conclusive evidence that thiamin has no efficacy against mosquitoe bites.