Well you are in the reactive near-field, so the antenna will de-tune, hence the power amplifier won't see it's ideal match, and not deliver as much power. All depends on antenna design and orientation, assuming they don't use active impedance tuning.

The base station also tells the phone to back-off. If the TX were running at full power, the battery will die quickly. Leave your phone in a metal, security cubby and your battery will die quickly; no power control and worse-case antenna loading.

Also the highly asymmetric data usage these days. Very little energy on the uplink.

FWIW I design antennas and amplifiers for a living. The only time I'd worry about RF exposure is to the cornea; that is a proven hazard. RF burns are a right of passage for PA designers, and are harmless as it's the outer skin layers.