The island is the major part of a Site of Special Scientific Interest[3][4] and some features have been given the designation of scheduled ancient monument.

The ruined pest house is situated on a flat area behind the dunes and further inland the religious complex is based at the foot of the hill.

[6] Additionally, further excavations of the site have uncovered decorated ridge-tiles and parts of the 12th century Purbeck Marble shrine that is thought to have once housed Saint Elidius relics.

It was constructed after an Act of Parliament in 1754 decreed that any plague-ridden ship north of Cape Finisterre heading for England should anchor off this island.

[22] The island is part of the St Helen's (with Northwethel and Men-a-vaur) Site of Special Scientific Interest (SSSI) and is designated for its rare flora and breeding seabirds.

[4] In 1940 the highest land on St Helen's was covered in maritime heath with ling (Calluna vulgaris) and bell heather (Erica cinerea) which was destroyed by fires from incendiary bombs from German aircraft during World War II.

At the time of the publication of Lousley's flora (1971) the vegetation had still not reverted to heath and by 1987 Hottentot fig (Carpobrotus edulis) had spread over much of the area.

Lousley found large areas of cliffs and rocks were also covered with Hottentot fig which is spread by gulls carrying the plant to their nests.

Around the pest house there is rank grassland and scrub with flowering plants such as hemlock (Conium maculatum), hogweed (Heracleum sphondylium) and good populations of balm-leaved figwort (Scrophularia scorodonia) which is spreading in one area.

[21] To allow access for visitors to the August church service, the tall vegetation is cut around the ruined buildings and on the path from the landing place.

On the north-west side of the island there is an area of maritime grassland with abundant thrift (Armeria maritima), sea beet (Beta vulgaris subsp.

[25] The fauna of the outer islands of the Isles of Scilly is generally poorly recorded because of, both the difficulty of reaching them, and the length of the time spent.

St Helen's has a long history of habitation and when Issac North visited the islands in 1850 he reported goats (Capra aegagrus hircus) and deer.

[29] Men-a-vaur (grid reference SV893175) is to the north-west of St Helen's and consists of three granite slabs covering an area of 0.55 hectares (1.4 acres) and rising to 38 metres (125 ft).

[14] Lousley recorded common scurvygrass (Cochlearia officinalis), tree mallow and a species of orache (Atriplex spp) although flowering plants are not a permanent feature.

[30] Northwethel (grid reference SV895163) is a 4.63 hectares (11.4 acres) and 15.2 m (50 ft) high island in Old Grimsby Channel, off Grimble Porth, Tresco.

A number of finds of flint, bone and pottery of unknown date have been deposited with the Isles of Scilly Museum on St Mary's.

[32] It is said that 1,500 Parliamentarians landed on the island in the mistaken belief they were on Tresco and a roughly rectangular enclosure with two sides of drystone walling is a possible ″Parliamentary army shelter″.

[23] Unusual plants (for the Isles of Scilly) in the brackish pool are sea-milkwort (Glaux maritima), fennel-leaved pondweed (Stuckenia pectinata) and red goosefoot (Chenopodium rubrum).