**Abstract**

The absorption of gamma rays is roughly proportional to the fifth power of the atomic number of an element. This immediately raises the issue of tissue ionisation enhancement effects from photoelectron production by elements of high atomic number incorporated into living tissue. The issue was raised in the 1950s in relation to calcium in the bone but has received little attention since then. New results, derived from mathematical modelling carried out at the University of Ulster, of photoemission from nanoparticles of gold and uranium are presented. These show that significant ionisation enhancement effects can occur when incorporated particles of high atomic number are exposed to natural background gamma radiation, effects which increase sharply at the lower energy end of the spectrum, around 150 keV. The effects must also occur for molecular species. The general problem is discussed, with reference to the literature, and approximate enhancement factors are derived for the effect. The implications for the evolutionary selection of elements by life are explored.

**Keywords:** gamma rays, ionisation, biological effectiveness, absorbed dose, radioprotection, photoelectron, secondary photoelectric effect, evolution
