Fly spray is a chemical insecticide

Insecticide

An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

 that comes in an aerosol can

Aerosol spray

Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. This is used with a can or bottle that contains a liquid under pressure. When the container's valve is opened, the liquid is forced out of a small hole and emerges as an aerosol or mist...

 that is sprayed into the air to kill flies

Fly

True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

. Fly sprays will kill various flying insects such as house flies and wasps.

Fly sprays often contain the powerful insect toxin Dichlorvos

Dichlorvos

Dichlorvos or 2,2-dichlorovinyl dimethyl phosphate is a highly volatile organophosphate, widely used as a insecticide to control household pests, in public health, and protecting stored product from insects. It is effective against mushroom flies, aphids, spider mites, caterpillars, thrips, and...

 which is often targeted by environmental groups as a carcinogen

Carcinogen

A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

ic compound

Fly sprays kill insects by interrupting the nerve signals from the motor neurons in the insect. This in turn causes them to go into a permanent contraction which makes flying and respiration

Respiration (physiology)

'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

 impossible, and the insects then die of asphyxiation.

Fly spray is dangerous to pets

PETS

PETS may be an acronym for:* Pet Travel Scheme, which allows animals to travel internationally without quarantine* Pets Evacuation and Transportation Standards Act...

 as well as fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

and should be used with caution.