Zappers are no good they kill more beneficial insects such as ladybugs plus many folks put them in the area they don't want these pests and guess what, it draws them in.
Another problem I have seen is folks sitting by a bug zapper or cooking by one and once the flies get zapped they are spewed everywhere and flies carry all kinds of nasty bacteria.
I researched this years ago, its better to use environmentally safe methods I use a fan if there's a mosquito problem.
If you are convinced on using a bug zapper, place it away from you
A bug zapper is a popular outdoor appliance which may be installed by a homeowner in an attempt to control the annoyance of flying insects. Its name comes from the characteristic “zap” sound emitted when an insect is electrocuted by the device. Around the yard of a home, they are primarily used to kill mosquitoes, which infect skin with an itchy bump, and can transmit the deadly diseases and viruses.
Bug zappers operate by luring flying insects with an ultraviolet light into an electrical current which electrocutes and kills them instantly. Flower patterns that tend to attract insects are better revealed in ultraviolet light, and so many flying insects that feed on flowers will be drawn to the bug zapper. However, they will come into contact with the wire mesh of the unit before the UV light, which completes the electrical circuit and disintegrates the insect.
Depending on location and placement, bug zappers can kill many thousands of flying insects per night.
Despite their widespread use, various studies question the effectiveness and safety of bug zappers. One of two main concerns is that mosquitoes and other biting insects are typically more attracted to the carbon dioxide (CO2) and water vapor in the breath of humans and animals than to the UV light. Therefore, they typically kill large numbers of harmless and/or beneficial insects, and ultimately fail to reduce the types of insects that prompted the purchase of a bug zapper in the first place.
A second concern raised by the numerous studies conducted on bug zappers addresses the side effect of the process: The electrocuted insects are blasted into a fine mist that contains miniscule insect parts, as well as some surviving bacteria and viruses. This mist can be spread up to 7 feet from the device, contaminating the air surrounding the zapper with potentially dangerous organisms commonly carried by flies. For this reason and for simply hygiene purposes, a bug zapper should never be placed near a food preparation area, in a hospital, or around any other sterile environment to prevent the potential spread of disease.
Children should not be allowed to play beneath an operating bug zapper to avoid the risk of coming into contact with these potentially dangerous organisms. There are some bug zapper models that contain a tray which catches insect debris, resulting in less of a health risk. However, the tray will not catch any of the mist that falls beyond its reach.