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Found this gem on G+:

important health tips

The rest seems all extremely circumstantial except the very last item. The 'radiation' they're talking about, I'm assuming, is electromagnetic radiation - which would be weaker by a low battery, not stronger.

Any evidence showing that cell phones emit more radiation while low on battery (say, below 20%)?

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They all sound like nonsense to me. And there should be no relationship between mobile phone battery and radiation levels: they depend on the signal strength from the base station. –  matt_black Nov 26 '12 at 13:11
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If anything, I'd have thought radiation from a phone would be stronger when reception is extremely weak, not battery level is extremely low. Maybe that's where the "last bar" could have come from, and it got changed in a game of Chinese whispers. –  Andrew Grimm Nov 26 '12 at 13:15
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You could play the correlation game: the times that your battery manages to get to the lowest bar between regular charges tend to be the times where you are in a weak reception area and the antenna needs to consume more power... yeah. So @AndrewGrimm is very likely right. How do we write an answer? –  Oddthinking Nov 26 '12 at 13:52
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Apologies for my ignorance in the matter, but it sounds like we're relatively equally exposed to radiation anyway (?) Either more from cellphone and less from tower when far away or less from cellphone and more from tower when closed by one. Not sure which one is more dangerous to our health (?) Is my perception too far off? Thanks- –  user12016 Mar 19 '13 at 20:23
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I doubt they confuse anything themselves, but they are deliberately trying to confuse readers in order to make people scared of "radiation". –  jwenting Mar 20 '13 at 7:07
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1 Answer

up vote 25 down vote accepted

The radiation is 1000 times stronger

According to Wikipedia The transmission power of a GSM handset is limited to a maximum of 2 watts in GSM 850/900 and 1 watt in GSM 1800/1900.

According to a Radio-Electronics.com article "GSM Power Control and Power Class" the base station controls handset power output in the range 2-19 which is 39 dBm to 5 dBm.

According to Wikipedia, 39 dBm is around 8 W and 5 dBm is 3.2 mW.

8 W is certainly more than a thousand times stronger than 3.2 mW.

There is some inconsistency in this information, but if we take 3.2 mW to 2000 mW to be the actual range, a 625 X range is still not too far from the claim.

when phone's battery is low

There is no direct causative link between handset battery charge level and handset transmit power level.

Regardless of battery charge, when you are standing next to a transmission tower your handset is only outputting the minimum power needed to communicate. Not the maximum.

As the above linked article should make clear, the power level of the handset is determined by the base station and is adjusted so that the base station can receive the handset signal clearly.

A low base-station signal level indicated at the handset will usually correlate with a need for the handset to transmit at higher power in order to reach through the distance or obstacle that is causing the weakness of the received base-station signal.

A low battery level may indicate only that you have been playing Angry Birds for too long, or forgot to charge your handset last night and may have absolutely no correlation to current effective distance between handset and base-station.

Conclusion

Whilst a 1000 fold variation in power is of the right order of magnitude, your handset's battery level indicator is not a useful indicator of the amount of "radiation" currently emitted by the handset (in talk mode).

The related claim that radiation from the handset may cause health issues is covered in a number of related questions:

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@Oddthinking: Thanks for editing your comment into the answer, beat me to it. –  RedGrittyBrick Nov 27 '12 at 0:23
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It looks to me like the post meant to say: Don't answer your call when the signal is on a single bar--not when the battery is on a single bar. –  Flimzy Nov 28 '12 at 8:26
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