A threshold effect means that some teratogens are virtually harmless until exposure reaches a certain level, at which point they become damaging.

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A threshold effect is a well-established phenomenon in teratology where a teratogen has no harmful effects until exposure reaches a certain level (i.e., the threshold), beyond which it becomes harmful to the developing embryo or fetus.

A Threshold Effect Means That Some Teratogens Are Virtually Harmless