Police and Local Resilience Partners are dealing with a "major incident" following the recovery of cannisters containing mustard gas at Roughton Woods near Woodhall Spa.
A man aged 38, from Lincoln, has been arrested on suspicion of being in possession of a noxious substance. He remains in custody.
The canisters were found on Sunday October 1 by two people who were taken to hospital and received treatment for burns and minor respiratory problems. They returned home the same day.
Work is ongoing to confirm the woodland is a safe place and that there is no further risk to people who visit the area. Police are working to trace anyone who may have been exposed to the substance, but authorities do not believe there is a wide contamination problem.
Superintendent Phil Vickers, from Lincolnshire Police, said: “We are working with other emergency services, the Environment Agency, Anglian Water, Public Heath England and the military to reduce the risk from the materials that have been found.
“The people who found it have been exposed to it and we are ensuring they have the best possible treatment.
“The work we are doing is to ensure nothing or no-one else is contaminated. There isn’t any need for alarm.
In a statement police reassured the public this is not a terror related incident.