The type in use in the US today: millimeter wave
Current US airport scanners use millimeter-wave technology, a form of non-ionizing radiofrequency energy — the same broad category as Wi-Fi, not X-rays. The EPA states these scanners emit far less energy than a cellphone.
The type that's been phased out: backscatter X-ray
An earlier generation of scanner used backscatter X-ray technology, which is ionizing radiation — a different physical category with an established, if very small, cancer-risk model at higher cumulative doses. The EPA confirms backscatter X-ray scanners are no longer used for passenger screening at US airports.
Why the distinction matters
Ionizing and non-ionizing radiation aren't different strengths of the same thing — they're different physical processes. Ionizing radiation carries enough energy to strip electrons from atoms, which is the mechanism behind X-ray cancer-risk models; non-ionizing RF, at the levels used in Wi-Fi or millimeter-wave scanning, does not.
If you'd still rather not go through one
At current US checkpoints, a pat-down search is available as an alternative to the millimeter-wave scanner if you prefer to opt out — no justification required.