Nuclear fusion is a process whereby the nuclei of atoms are joined, or fused,  and in which energy is released. One of the by-products of fusion is helium-4 gas. A recent fusion experiment was conducted using heavy water contained in a sealed flask. The flask was, in turn, contained in an air-filled chamber designed to eliminate extraneous vibration. After the experiment, a measurable amount of helium-4 gas was found in the air of the chamber. The experimenters cited this evidence in support of their conclusion that fusion had been achieved.
The statement 'The amount of helium-4 found in the chamber's air did not exceed the amount of helium-4 that is found in ordinary air' would cast doubt on the experimenters' conclusion.