The level of dioxins - a group of carcinogenic chemicals - in soil and vegetation has fallen by about 70% over 20 years, a UK-wide survey suggests.
Industrial emissions will rain out of the atmosphere and go into soil
The drop may reflect more stringent environmental regulations for industry - formerly a key producer of dioxins - which came into force in the mid-1980s.
The Environment Agency analysed soil and grass samples from 200 sites across the UK for pollutants.
It said it provided the first "national snapshot" of soil contamination.
Declan Barraclough, project manager of the UK Soil and Herbage Survey, said: "One of the inadvertent side-effects of human activity is the increased release of chemicals into the environment.
"We are very familiar with greenhouse gases, but there are other more complicated organic chemicals that are released into the environment and many of these accumulate in soil."
The Environment Agency scientist added: "We need to protect soils by monitoring the concentration of these toxic persistent contaminants."
The survey, which looked at 86,000 data-points across the UK, tested soil and vegetation for several chemicals, including:
- Dioxins - a by-product of combustion of organic matter and production of chemicals and pesticides
- Polychlorinated bi-phenyls (PCBs) - a group of man-made chemicals formerly used in industry, but now banned in many countries since the 1970s
- Polycyclic aromatic hydrocarbons (PAHs) - by-products from the combustion of organic compounds like petrol
"These are some of the bad-boys of the chemical world," said Dr Barraclough. "They are toxic to us and wildlife, and they are persistent in the environment, and we really don't want them around."
The researchers compared the results of their survey with previous soil studies, including analyses of archived soil samples taken from 1843 at the Rothamsted Research in Hertfordshire.
The soil archives showed that dioxin levels had increased steadily since the 1850s, in line with industrialisation.
But the latest test result from the survey revealed that levels had now fallen sharply, by about 70%, since the mid-1980s.
This coincided with the implementation of the National Atmospheric Emissions Inventory, which regulates the amount of dioxins produces by industry, Dr Barraclough said.
"This is hugely important; it means by regulating dioxin emissions we have reversed an upward trend that went on for more than 100 years," he added.
The survey also found both urban and industrial areas were still sources of PCBs, even though PCB production was banned in many countries in the 1970s because of the chemicals' toxicity.
Levels of PCBs have fallen 800-fold since the restrictions were introduced, explained Dr Barraclough. But the fact that they were present in higher concentrations in some areas suggested they were still leaking into the environment, potentially from sources such as older PCB-containing building material or transformers.
Dr Barraclough said the results showed that scientists now had to tackle these remaining sources.
The survey also revealed that PAHs were still persistent in soil, but were in higher concentrations in urban and industrialised sites where road traffic - now considered to be the main producer of these chemicals - was heaviest.
The results of the survey will be used to inform future government policy on contaminants and they will also provide a baseline for assessing future trends.