Giant hogweed is growing along the Loop path between the roundabout under the A13 and the crossing of Coldharbour Lane. There are large plants on either side of the path along this stretch. The Woodland Trust gives full details but warns specifically that the sap of giant hogweed can cause burns. The sap contains a chemical which makes skin extremely sensitive to sunlight. If the sap gets onto your skin, then you are exposed to sun, your skin can blister badly and blistering can recur over months and even years.
The Woodland Trust's advice is that best way to avoid injury is to familiarise yourself with the plant. Avoid brushing through patches of giant hogweed and exposing yourself to plants which have been cut which might cause you to get sap on your skin. As the plants get bigger during the summer this could be quite tricky for walkers given that the Loop path is quite narrow where the plants are growing.