classified as "non-toxic." However, a later study showed PCB concentrations as high as 5,797 µg/kg, which is above published threshold levels for such chlorinated compounds. A comparison between individual PAH compounds that have different thermal stabilities shows that the source of PAH pollution in the Clyde is different in different parts of the river. PAH in the inner Clyde (Cuningar to Milton) are from combustion sources (vehicle exhaust, coal burning), whereas PAH in the outer Clyde are from petroleum spills. The amount and type of sedimentary pollution in the Clyde reflects the area's industrial history. In order to assess how the nature of the pollutants has changed over time, from 1750 to 2002, seven sediment cores of one metre's depth were collected, and dated using lead concentrations and changing lead isotope ratios. The sediments showed a long but declining history of coal usage and, beginning around the 1950s, an increasing reliance on petroleum fuels. The decline of hydrocarbon pollution was followed by the appearance of PCB concentrations in the 1950s. Total PCB concentration levels peaked in the period 1965 to 1977, and declined beginning in the 1990s. The Polmadie Burn, which flows into the Clyde at Richmond Park, remains heavily contaminated by hexavalent chromium, to the extent it turned bright green in 2019, and yellow in April 2021. Although pollution from heavy industry and power generation has been decreasing, there is evidence that man-made pollution from new synthetic compounds in electrical products and textiles has been increasing. The amounts of 16 polybrominated diphenyl ether (PBDE) compounds used as flame retardants in televisions, computers, and furniture upholstery were measured in sediment cores collected from six sites between Princes Dock and Greenock. Comparison of the amounts of PBDE compounds revealed a decline in certain compounds, in line with the European ban on production of mixtures containing environmentally harmful PBDE with eight and nine bromine atoms. At the same time, there was an increase in the amounts of the less harmful mixture, composed of ten bromine atoms. Media The Clyde plays an important role in the Para Handy novels of Neil Munro, and subsequent adaptations. It also features in novels by Alasdair Gray, Matthew Fitt, and Robin Jenkins. It is mentioned in the "Ossian" poetry of James Macpherson, as well as the works of John Wilson, William McGonagall, Edwin Morgan, Norman McCaig, Douglas Dunn and W.S. Graham. It also features in the work