Generally, for a child's exposure to secondhand smoke to become a custody issue, it must be shown that: 1) the child suffers from some medical problem; and 2) this problem is caused or exacerbated by secondhand smoke. To date no cases have restricted a parent's smoking or based a custody decision on a parent's smoking absent these two factors.

The hypothetical harm to the child alone has not been sufficient to make ETS an issue in custody cases. However, as seen in Unger, the medical problem affecting the child need not be severe. In Unger the only symptom of an adverse impact from ETS was simply a persistent cough. It is more common for asthma, allergies, and respiratory infections to be established as the adverse health effects from exposure to ETS necessary to make smoking an issue in custody matters.

Asthma is a symptom that courts have found is exacerbated and in some cases caused by ETS. In Gilbert v. Gilbert, 1996 WL 494080, at *5 (Conn. Super. Ct. 1996), a Connecticut trial court found that "[t]he reported medical evidence seems clear on the serious deleterious effects of smoking on an asthmatic child. For example, the presence of one or more smokers in an asthmatic child's household resulted in 63% more emergency room visits than visits of asthmatic children from nonsmoking households."

Once the child's medical problem has been demonstrated, the focus then turns to causation. In most cases expert testimony is necessary to link the child's medical problem to the parent's smoking. Unless this testimony clearly satisfies the court that the child's exposure to ETS is the cause of the harm, the issue of smoking may become nullified. For example, in Re Marriage of Diddens, 625 N.E.2d 1033 (Ill. App. Ct. 1993), the children suffered from various respiratory infections, and they were exposed to smoke in the mother's home where they resided. The father sought to modify a custody arrangement based on the adverse health effects of ETS. Notwithstanding these facts, the court refused to modify the custody arrangement stating "it was not established that the environment in the [mother's] home caused the health problems."