  This work considers the problem of energy efficiency maximization in a
RIS-based communication link, subject to not only the conventional maximum
power constraints, but also additional constraints on the maximum exposure to
electromagnetic radiations of the end-users. The RIS phase shifts, the transmit
beamforming, the linear receive filter, and the transmit power are jointly
optimized, and two provably convergent and low-complexity algorithms are
developed. One algorithm can be applied to the general system setups, but does
not guarantee global optimality. The second algorithm is provably optimal in a
notable special case. The numerical results show that RIS-based communications
can ensure high energy efficiency while fulfilling users' exposure constraints
to radio frequency emissions.
