8 month old has one chest X-ray in the ER because of high fever. His lungs were clear. Is he now at risk for cancer?

Not to worry. One chest xray is very minimal radiation to be worried about cancer risk in a baby, so do not worry there very little radiation and no risk of baby getting cancer from one chest xray.
Minimal risk. Your concern is understandable. Radiation exposure from a chest xray is minimal. For an adult (higher dose than children) a chest xray dose is equivalent to 10 days of natural background radiation (yes there is radiation exposure around us all the time).
No. Medical use of x-+rays is considered pretty safe. Morover the amount of radiation exposure from a chest x-ray is miniscule. Therefore there is no reason to think he is at risk for devloping cancer due to the x-ray by itself.

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