Erämetsä was an assistant geologist for the Geological Commission and made five exploration trips to Lapland.
He carried out a number of diamond surveys in the 
Paatsjoki area of Petsamo in the 1930s, trying to  verify reports of diamonds in the sands in that area. He concluded that what had been discovered was likely to have been spinel octahedra.
In 1939 Erämetsä and Thure Georg Sahama developed a new method for the separation of rare earths using chromatography. Their chromatographic separation method became a standard for the separation of rare earth and radioactive actinides into ion exchange resins.
Erämetsä worked as a lecturer in analytical chemistry at the Helsinki University of Technology (TKK) from 1940 to 1946 and then as a professor of inorganic chemistry from 1947 to 1973. During his time there, the department received new hardware including a spectrograph, Finland's first equipment for spark source mass spectrometry, and X-ray fluorescence equipment.
Large-scale industrial production of rare earths occurred in the 1960s at the Typpi Oy mills in Oulu, where lanthanide ores of the Kola Peninsula were being refined. 
Erämetsä's most well-known, though controversial achievement, was the isolation of the element promethium  from these natural sources. In 1965, he published a report stating that in 1964 he had successfully isolated isotopes of promethium from natural ores.
Erämetsä separated out traces of Pm from a rare earth concentrate purified from apatite, 
resulting in an upper limit of 10 for the abundance of promethium in nature; this may have been produced by the natural nuclear fission of uranium, or by cosmic ray spallation of Nd.
The international scientific community was initially suspicious of the findings, as promethium had previously only be produced as a degradation product from the fission of uranium ore. Due to promethium's short half-life, its concentration in natural minerals was thought to be too small to be isolated. Subsequent findings were in line with those of Erämetsä.
Erämetsä and Allan Johansson used praseodymium to experiment with the development of a solid electrolyte fuel cell.
Erämetsä studied the presence of trace elements including rare earths in soils and plants such as lichens and mosses. He also studied of the presence of rare earths in the human body, over a period of many years. Results were negative from 1945 to 1969, when x-ray emission spectroscopy was introduced. With the new instruments, the researchers detected yttrium and other lanthanides. In one study, autopsies were performed on the bodies of people who had died in Helsinki hospitals.  Yttrium was found in varying amounts and locations in the bodies of almost half of those examined (37/80).
Erämetsä was concerned with epidemiologic evidence of environmental factors affecting human health, cf. the presence of metals in drinking water and their possible relationships to coronary heart disease.