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.


Why would a sauna company increase confusion this way? What do they actually mean?

I know this company knows the difference because I have explained it to their CEO directly.  This company sold saunas for years as "low EMF" that only had magnetic field mitigation, and had no electric field mitigation  These saunas expose users to high electric field EMF even though they were sold as "low EMF".  As an engineer reading their current material, it is apparent to me that by "low ELF" they actually mean "lower ELF electric field EMF" (this is also clear from reading their third party EMF report).  In other words, they have added some mitigation of the electric fields radiating from their product.  When they say "low EMF", they actually mean just mitigation of ELF magnetic fields.
Unfortunately, this ignorant or intentionally misleading language has been picked up by other sauna companies that now say "low EMF" when they only mean ELF magnetic field mitigation.  This dupes customers into thinking that they are actually buying a "low EMF" product, when they are not.  To be clear, these companies are making a fraudulent claim because EMF refers to BOTH electric and magnetic fields.

High Tech Health Saunas Have the Lowest Total EMF

Our saunas have the best ELF magnetic field mitigation and the best ELF electric field mitigation and our third party heater measurements demonstrate this.  So, in other words, High Tech Health sells a TRULY low EMF product.  Other companies have done partial EMF mitigation at best.
Any company that does not use these terms correctly probably also does not understand and implement the technical details of these claims.
Low ELF is not a thing.