Hydrogenation is the process of making certain unsaturated fats or oils more saturated such that they have a higher melting point and a longer shelf-life. If not completed hydrogenated, some unsaturated fats turn into trans-unsaturated fats. This partial hydrogenation is performed when the desired effect is a product that is solid at room temperature but melts upon baking or consumption.
Trans fats, when eaten, increases one's risk of coronary heart disease by raising LDL ("bad") cholesterol and lowering HDL or "good" cholesterol.
In order to emulsify the oils and other ingredients with water, companies should hydrogenate instead of using synthetic emulsifiers such as PEG, which also have very negative health effects. During this process, companies should ensure that the oil is completely hydrogenated so that NO TRANS FATS are produced. Hence hydrogenated oils in products will not affect levels of LDL or HDL if consumed.
Most hydrogenated oils reviewed on the Skin Deep database have a safety ranking of 0 (low hazard).