Cell Tower ERP a Specific Case
Almost certainly not an issue, but without knowing the details of the antenna design all you can do is measure or compute worst possible design values... 

Consider that a 16.5dBi (I am guessing dBi, but you don't specify) omni directional antenna will have a very 'flat' pattern (It kind of must if it is to have 16.5dB of gain while being an omni), and it is 30m up, so both the pattern and the 30m between you and the bottom of thing thing makes for low field strengths close to the antenna. 

At ~500m (Use SI units already, makes the sums easier), with an EIRP of 120W * 10^(16.5/10) = ~5,400W EIRP, what power density do we have if you were right in the radiating plane (Probably a slightly tilted down cone, but whatever)? The area of a sphere is 4*Pi*R^2, so at 500m that energy is spread over 3.14*10^6 m, giving a power density of 1.7mW/m^2, well below any sane exposure limit. In W/square cm that is 170nW/cm^2. 

How about a really cack handed design that put 120W out in a isotropic radiator (stupid, and a waste because an isotropic radiator by definition cannot have gain, which is why nobody does it), how much power would someone standing ~30m away (at the base of the tower) be exposed to? 120W / 4 * 3.14 * 30m^2 = ~10mW/m^2 (~1uW/cm^2).

If you are going to worry about this stuff, worry about the 2W transmitter in your pants pocket, at 10cm (0.1m), that thing works out to (assuming isotropic radiator, not a safe assumption), 2W / 4 * 3.14 * 0.1^2 = 16W/m^2 (about 10,000 times what that tower is hitting you with at 500m). Of course it throttles back when it has a good signal from a reasonably nearby tower, so you might see the tower as lowering your exposure in some circumstances....