Families enjoying an afternoon on Gyllyngvase Beach on Sunday had to clear the sand after a woman suffered a severe allergic reaction to a weever fish sting and had to be air-lifted to hospital.
The 20-year-old woman from Watford lost conciousness after stepping on the fish in the shallows.
Weaver fish, which are common along the Cornish coast, raise spines along their backs when they feel threatened, and can give a painful sting, although severe reactions are rare.
Falmouth Coastguard Rescue Team were called to help Gyllnygvase RNLI lifeguards just after 1pm after the "severe allergic reaction".
The search and rescue helicopter R193 was requested due to the condition of the casualty and landed on the beach as the Coastguard team arrived, and a cordon was thrown up around the beach to create a landing zone.
Eyewitness Russell Shrimplin, a father enjoying a birthday party with his children on the beach, said they had to clear everything away to make space for the helicopter.
"The first we knew was when the RNLI lifeguards started telling people to clear the beach. They put a red flag in the sand and it was only when the the helicopter arrived that we knew there was a person who needed airlifting."
The casualty was taken to the Royal Cornwall Hospital at Treliske.