The compounds - known simply as 'dioxins' - are persistent, bioaccumulative and toxic by-products of combustion and chemical manufacture.
The conclusions stem from measurements of the compounds deposited on grass which is cut and stored as part of the Park Grass experiment at Rothamsted research station in Hertfordshire.
Samples from the same unfertilised plot have been cut and stored since the mid-nineteenth century - making them an ideal subject for studying long-term trends in dioxin deposition.
Previous reports from the team in Lancaster - headed by Dr Kevin Jones - have highlighted the decline in deposition since the 1970s. The latest paper shows that the decline has continued.
"This bodes well for future trends in human exposure," the authors conclude, since there is a time lag before reductions in environmental levels make their way through the food chain.
The paper also discusses the past pattern of dioxin levels and likely sources, looking at the changing profiles of congeners since 1900.
In summary, dioxins from poorly controlled combustion process such as coal fires and industrial plant dominated the pattern until the 1950s.
Dioxin levels started to decline following clean air legislation and a switch to other fuels. However, contamination from chemical industry sources, particularly pentachlorophenol (PCP) biocide manufacture and use increased, peaking in the early 1960s.
With a ban on PCP and a continuing shift to cleaner combustion, levels continued to decline. Industrial pollution control legislation and Directives on waste incineration further reduced emissions.
A recent review commissioned by the Environment Department concluded that garden bonfires and small uncontrolled combustion processes were now likely to be the principal source of UK dioxin emissions (ENDS Report 378, pp 18-19 ).