To examine the requirements of a medical monitoring claim, we first must look to our Supreme Court's decision in Potter v. Firestone Tire & Rubber Co. (1993) 6 Cal. 4th 965, 863 P.2d 795. The court explained that "in the context of a toxic exposure action, a claim for medical monitoring seeks to recover the cost of further periodic medical examinations intended to facilitate early detection and treatment of disease caused by a plaintiff's exposure to toxic substances." (Id., at pp. 1004-1005.) It ultimately held that "the cost of medical monitoring is a compensable item of damages where the proofs demonstrate, through reliable medical expert testimony, that the need for future monitoring is a reasonably certain consequence of a plaintiff's toxic exposure and that the recommended monitoring is reasonable. In determining the reasonableness and necessity of monitoring, the following factors are relevant: (1) the significance and extent of the plaintiff's exposure to chemicals; (2) the toxicity of the chemicals; (3) the relative increase in the chance of onset of disease in the exposed plaintiff as a result of the exposure, when compared to (a) the plaintiff's chances of developing the disease had he or she not been exposed, and (b) the chances of the members of the public at large of developing the disease; (4) the seriousness of the disease for which the plaintiff is at risk; (5) the clinical value of early detection and diagnosis. Under this holding, it is for the trier of fact to decide, on the basis of competent medical testimony, whether and to what extent the particular plaintiff's exposure to toxic chemicals in a given situation justifies future periodic medical monitoring." (Id. at p. 1009.) Potter emphasized that "toxic exposure plaintiffs may recover 'only if the evidence established the necessity, as a direct consequence of the exposure in issue, for specific monitoring beyond that which an individual should pursue as a matter of general good sense and foresight.'" (Potter v. Firestone Tire & Rubber Co., supra, 6 Cal. 4th 965, 1009.) All four plaintiffs in Potter were long-term cigarette smokers, and the defendant complained that it should not be saddled with financing their long-term health care needs as a result of their voluntary exposure to smoking. The court responded that "there is no question that a defendant ought not to be liable for medical monitoring of a plaintiff's preexisting condition that is unaffected by a subsequent toxic exposure," but stated that the defendant may be held responsible for "any increased or different monitoring of the preexisting condition (whether or not the preexisting condition is caused by the plaintiff's voluntary conduct) where necessitated as a direct result of the subsequent exposure." (Id., at p. 1009, fn. 27.) "Even if a defendant negligently exposes a smoker to toxins that significantly increase the smoker's risk of cancer, that defendant is not liable for reasonably certain future medical monitoring costs unless the recommended monitoring calls for tests or examinations that are in addition to or different from the type of monitoring that the smoker should prudently undertake regardless of the subsequent toxic exposure. However, if additional or different tests and examinations are necessitated as a result of the toxic exposure caused by the defendant, then the defendant bears full responsibility for their costs. The costs of additional or different monitoring made necessary by the defendant's conduct should not have to be shared by the plaintiff since the plaintiff already remains responsible for any monitoring that is shown to be medically advisable due solely to his or her smoking or other preexisting condition." (Id., at p. 1012, fn. 31.)