Being a heterozygous carrier (having a single copy of ΔF508) results in decreased water loss during diarrhea because malfunctioning or absent CFTR proteins cannot maintain stable ion gradients across cell membranes. Typically there is a build-up of both Cl− and Na+ ions inside affected cells, creating a hypotonic solution outside the cells and causing water to diffuse into the cells by osmosis. Several studies indicate that heterozygous carriers are at increased risk for various symptoms. For example, it has been shown that heterozygosity for cystic fibrosis is associated with increased airway reactivity, and heterozygotes may be at risk for poor pulmonary function. Heterozygotes with wheeze have been shown to be at higher risk for poor pulmonary function or development and progression of chronic obstructive lung disease. One gene for cystic fibrosis is sufficient to produce mild lung abnormalities even in the absence of infection.