Food can influence the health of the chemically sensitive patient and patient with chronic degenerative disease in many ways. Factors that contribute to the alteration of the nutrient value of foods include cultivation of limited genetic variance of foods, monocropping and mechanical disruption of the soil, and the use of artificial fertilizers, herbicides, and pesticides. Food grown in contaminated waters can cause severe illness. Molds and their mycotoxin metabolites whose total load can cause food sensitivities, various types of pesticides, food additives, and gas, toxic foods, solvents, hydrogen sulfide, and some toxic metals such as mercury have been noted to instigate sensitivity-related illnesses. The associations of polychlorinated biphenyls (PCBs) and methylmercury exposures with indicators of allergy and allergic disease may involve both stimulation and inhibition of immune system functions. However, the interactions of PCBs with brain endothelium and the blood-brain barrier in vivo are virtually unknown.