What exactly is “low ELF”? Do High Tech Health saunas have “low ELF”?
ELF stands for "Extremely Low Frequency".
ELF refers to a range of frequencies that include the electric power grid's frequency, which is 60 Hertz in the United States. See the wikipedia page for more detail. "ELF" is used to describe magnetic fields (as in "ELF magnetic fields") and electric fields (as in "ELF electric fields"), such as those that result from any consumption of power grid electricity. All appliances that are plugged into the wall for power will radiate ELF electric and ELF magnetic fields (jointly considered EMF, or simply, "electromagnetic fields").
As an electrical engineer, I am annoyed when I see a particular sauna company advertise a sauna with "Low EMF and Low ELF". "Low ELF" might refer to an unhappy elf, or it might refer to a short elf, but it does not actually mean anything. This misuse of terminology does a tremendous disservice to customers who are trying to understand these concepts.