I'll have to admit my memory isn't what it used to be.
"Pollutants from burn barrels vary depending on the type of waste materials burned but, typically, emissions include dioxins, ash, furans, halogenated hydrocarbons, carbon monoxide, lead, barium, chromium, cadmium, carbon dioxide, sulfur dioxide, arsenic or mercury. Burn barrels also often emit acid vapors and carcinogenic tars.
Pound for pound, garbage burned in a burn barrel gives off twice as many furans, 17 times as much dioxin, and 40 times as much ash as a municipal incinerator. For example, a 1997 EPA study showed that as few a two households burning garbage produce as much dioxin as a 200 ton/day municipal incinerator operating with air pollution controls. Also, municipal incinerators operate at 2,200 degrees F to insure complete combustion, and they use efficient filters to reduce harmful emissions.
Burn barrels emit more pollutants because they operate at relatively low temperatures (400-500 degrees F), resulting in incomplete combustion of the wastes being burned. They also are less efficient at combustion and emissions are concentrated close to the ground, thus creating a greater risk of direct exposure to harmful pollutants. The closer you stand to the burn barrel, the more of these harmful chemicals you may inhale.
Residual ash is another result of incomplete combustion. Frequently, a significant portion of material in the barrel - especially at the bottom - is not burned up. Ash disposal outside of a sanitary landfill can cause problems.
Also, ash particulates can irritate the eyes and throat and can restrict visibility. In addition, ash can damage the lungs, cause bronchitis, emphysema, and lung cancer, and can seriously affect people with asthma or certain allergies. Ash also contains heavy metals that may seep into groundwater." Wisconsin Department of Natural Resources website