Most courts must look at all factors related to the "best interests of a child" in making a custody decision. At least one court has found a custodial parent's smoking to be a relevant factor in changing custody. In that Georgia case, the child was diagnosed with asthma after the divorce and the mother continued to smoke. The court held that the mother's continued smoking in the child's presence showed inadequate concern for the welfare of the child.
This, alone, may not be enough to require a change in custody, but it doesn't help. If your ex has shown an intention to use your smoking against you, stop smoking around the child. Better yet, stop smoking.
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