The chemistry of Giant Hogweed and how it causes skin burns

Andy Brunning

7 years ago

Giant Hogweed Skin Burns Chemistry — Click to enlarge

Brushing past a plant in the undergrowth whilst out walking might sound fairly innocuous. In the case of Giant Hogweed, however, it’s anything but. This plant’s sap can cause burns and blistering after contact, and there’ve been an increasing number of articles warning of the danger it poses over the past few months. Here, we take a look at the chemical compounds behind the sap’s unpleasant effects.

Giant Hogweed is a plant that was originally native to Central Asia. However, it made its way to the UK as a result of it being brought over for display in ornamental gardens, and it has subsequently reached a number of other countries in Europe, as well as the United States and Canada. Unlike some other imported plants, such as Rhododendrons, it doesn’t pose much of a threat to indigenous plants, but it can have some nasty consequences for humans if we come into contact with its sap.

Giant Hogweed sap contains, amongst other constituents, a family of compounds known as furanocoumarins (sometimes also referred to as furocoumarins). These compounds are the key to the effects of Giant Hogweed sap. They are found throughout the entire plant: the flowers, the stems, the roots, the seeds; but they are found at their highest concentrations in the leaves.

Their phototoxic effects are due to their interaction with DNA when they absorb UV light of a specific wavelength. The sun gives off UV radiation as well as visible light – which is the reason we have to slap on the sunscreen in the summer – and UV radiation with a wavelength between 320-380 nanometres can trigger the effects of furanocoumarins. This falls into the band of UV radiation referred to as UV-A radiation (320-400 nm) which accounts for 95% of the UV radiation from the sun that reaches the Earth’s surface.

Furanocoumarins can react with our DNA when exposed to UV-A radiation. DNA is made up of several components (there’s more detail on its composition here), and one set of components are the bases that help hold DNA strands together. Furanocoumarins can react with these bases to form adducts, damaging it. These adducts can then go on to react with other bases in the DNA strands on exposure to further UVA radiation, resulting in crosslinks between strands. Ultimately, this can cause cell death, and results in the characteristic skin reddening and blisters seen on exposure to Giant Hogweed sap.

Some cases of exposure to the sap have been so severe that those exposed to it have required hospitalisation. The blistering can take up to 48 hours to appear, and so measures can be taken if contact with the sap is suspected. It’s advised that any affected areas should be washed thoroughly with soap and water, and any skin that has been exposed to the sap should be protected from sunlight for several days. It’s advisable to seek medical treatment if any skin inflammation subsequently occurs.

In the UK it is illegal to plant or facilitate the spread of Giant Hogweed and councils take active measures to remove it when its presence is reported. Despite this its numbers appear to have increased over the past few years, with increasing numbers of articles citing cases of Giant Hogweed burns. The Botanical Society of Britain and Ireland has a map showing where Giant Hogweed has been reported. A similar map for the USA and Canada appears here, though it is only based on site and county level reports so is by no means exhaustive.

If you happen to find Giant Hogweed growing in your garden, it’s a good idea to seek a professional exterminator to remove it rather than try to remove it yourself. If you come across it out in the wild, in the UK at least you can contact your local council to notify them of its presence.

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