Because door handles are commonplace and are interacted with by large numbers of people on a daily basis, they present opportunities for growth of bacterial colonies.
Many factors determine the habitability of a door handle for a population of bacteria. The material of the handle itself contributes to the growth of bacteria, with most door handles being constructed of stainless steel – a suitable home for bacteria. The material can affect the time bacteria can survive on door handles, but more important is the temperature and humidity of the environment. A hospitable environment can allow bacteria to thrive for anywhere from a few hours to a few weeks on a surface.
A crucial factor in the bacterial growth and spread is the location of the handle. A low-touch handle, such as that of a rarely opened closet will have significantly fewer bacteria than the high-touch handles in a busy public bathroom due to the frequency of contact with people's hands. Door handles in locations where elderly people or immune compromised people frequent should be cleaned frequently.