Ford Repair: Leaking Manifold, exhaust manifold, own health

Question
QUESTION: What are the possible effects to the engine from having a leaking manifold?

ANSWER: Intake or exhaust manifold?

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QUESTION: we are not entirely sure. A mechanic just told us the manifold. We do however think that it may be the exhaust because we cannot drive with the windows rolled up due to exhaust coming into the van when we drive.

Answer
Depending on the location and severity of the leak, it could be harmless or problematic. More importantly, consider you're own health. Carbon monoxide is very hazardous to you're health. To answer you're question, it is possible that you could bend a valve, depending on the severity of the crack or leak.