Incineration of plastic waste in an open field is a major source of air pollution. Most of the times, the Municipal Solid Waste containing about 12% of plastics is burnt, releasing toxic gases like Dioxins, Furans, Mercury and Polychlorinated Biphenyls into the atmosphere. Further, burning of Poly Vinyl Chloride liberates hazardous halogens and pollutes air, the impact of which is climate change. The toxic substances thus released are posing a threat to vegetation, human and animal health and environment as a whole.
So if the study is on municipal solid waste overall, being burned in a pit, it's probably not very relevant to burning plastics by themselves. Municipal waste contains all sorts of things you wouldn't want to burn, like phones, batteries, fluorescent light bulbs etc.