Here is another elementary REBUS for you to solve. As the Caption states, it is Something Very Necessary.
Be Fast ;)
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The answer is
TRUST. As @Lawrence has outlined, HIJKLMNO is H2O, also known as water, and FE is iron. These are chemical symbols, which fits the 'Elementary' hint.
When water interacts so intimately with iron, the iron corrodes to form RUST. Append that to the T in front and you get TRUST!
Why is that necessary? Because humans are social creatures, and without it, our most intimate relationships will corrode, just as the water corrodes the iron....
Apologies about the formatting! Am on my phone.
Tritium (T) in heavy water - said to be a "radiation hazard when ... ingested via ... water".
The letters T is on the outside, with FE interspersed among HIJKLMNO, i.e. H to O (H2O = water). Iron (Fe) with water could be considered heavy water. Heavy water normally contains deuterium, but tritium is hydrogen-3 is arguably even heavier.
It's elementary because
the answer relates to element symbols.
It's "very necessary" because
H2O (water) is necessary to ongoing life.
And "be fast" because
T is toxic. On the outside, T is not considered dangerous because "its beta particles are unable to penetrate the skin". Ingested via water, though, it's a radiation hazard.