The effect was negligable only when compared to what happened next.

There have been over 300 atmospheric nuclear blasts since 1948.
Some of them were above the Stratosphere and ionoshere, some were deep in the sea off San Diego. America was not the sole atmospheric H bomb tester.

The leukemia rate for children went up 1000%
A single atom of plutonium in your body has the ability to trigger a cancerous response.

The Kennedy administration was the last known presidency to have tested above ground.

Chernobyl radiocativity did make it worldwide but was worst to the north and west of the plant as far as Denmark and China.