Frequencies and power levels - is RFID safe?
RFID uses electromagnetic waves to communicate. The visible light that surrounds us everyday is, itself, an electromagnetic field, and one with a far higher frequency than that used for RFID purposes. The energy of an electromagnetic wave is proportional to his frequency. Thus, it is significantly more dangerous to be exposed to the high frequency ultraviolet light from the sun or sunbeds (called ionising radiations - known to penetrate the skin and cause damage to the DNA) than to the relatively low frequencies emitted by RFID.
The concerns are mostly related to the amount of heat generated in the human body as consequence of a direct exposure to electromagnetic waves and its potential effect on health - after all, we all know you can cook with microwaves. The amount of heat is dependent on the frequency (with microwaves winning hands down) and proportional to the power emitted.
However, the heating effect has proven to be relatively small: an RFID system uses powers which are hundred of times inferior to any microwave oven and comparable to a mobile phone. Also, the results of a recent ten-years long study recently released found no conclusive link between health and mobile phones - which use UHF and MW frequencies (emitted power is heavily regulated by the law, and kept at a conservatively very low level) . Basically, no one has been able to prove (yet) that prolonged exposure to low frequency electromagnetic fields is harmful, and the general consensus is, if there is an effect on health, it is statistically insignificant.
Privacy - can RFID be misused to track me/violate my privacy?
There is a diffused fear RFID can be used to track people and collect personal data without the user's consent.
In fact, many of the technologies we use every day rely on some sort of user identification. Mobile phones, Internet, credit and debit cards, store loyalty cards - every "action" is logged somewhere, often accompanied by "extra" information (both credit card logs and mobile phone records can supply a very good estimate of a person movements and daily habits).
So, the general consensus is that, while RFID could effectively be used to track people, it would be massively expensive and inefficient if employed on a large scale, while other technologies already offer it at virtually no added cost. Even the best active RFID technology covers at most 100 metres (which would require and unreasonable amount of investment) - while mobile phone antennae cover kilometres and we take extreme care of keeping our mobile always charged and turned on all the time.