Some warmers become very hot and can burn you when handled exposed skin. Be careful when touching warmers. If it seems like there is a danger that it will burn you, use a cloth to grab it.
People with sensitive skin sometimes react to the chemicals found in air activated and reusable liquid to solid warmers. If you know that you are susceptible to these types of reactions try to avoid using these products. If that is not possible, avoid making direct contact between your skin and the warmers.
Finally, if you chose to use a fuel-powered hand warmer, be extra careful. This style of hand warmer does not use an open flame but does utilize flammable gas and live coals. If you open the products while they are burning, do so carefully and in a controlled setting.