http://www.apple.com/legal/rfexposure/iphone9,1/en/

This is a link to Apple’s website that explains the radio frequency exposure, SAR (Specific Absorption Rate) level and steps to take with your iPhone, (these steps can be applied in large part to any phone). This page is referred to in the owners manual, which I am sure everyone has read.  I am going to highlight some key items that you should be made aware of and why.  Anything in bold in this post was just copied and pasted from the linked page above.

Carry iPhone at least 5mm away from your body to ensure exposure levels remain at or below the as-tested levels.

What you should know is that 5 mm is a little less than 1/4 inch, and if you do the conversion is pretty close to 1/5 inch.  This means that if you carry your iPhone in  in your front pocket, your back pocket, your bra, your inside coat pocket, or possibly even your jacket pocket you will be closer than the 5 mm that the manufacturer recommends.  This of course means that you are not using the product as intended and therefore if you were to be harmed by the said product you are going to be hard pressed to receive compensation.   This will also make it hard to call someone since in fact you need to hold the phone.  You might ask ‘how do you carry it then’ and the answer is, when you carry the phone make sure it is in airplane mode.  You will also need to turn off Bluetooth, location, and Wi-Fi (most of these will be turned off when airplane mode is on).  If all of those are turned off, carrying your phone becomes safer.

Cases with metal parts may change the RF performance of the device, including its compliance with RF exposure guidelines, in a manner that has not been tested or certified.

Metal will reflect or block transmission signals.  If your phone is trying to acquire a signal, and you have it in a metal case, it will have to boost its power in order to do this.  This means that you are going to absorb more radio/microwave frequencies.  This of course will mean that you are above the SAR limit that was deemed ‘safe’.  If your iPhone is currently in a metal case, and you plan to be around it, please reread and follow the italics section above.   It would also good to consider a plastic or rubber case, instead of metal.

Here are some other important things to know or consider:

• The testing for SAR of a phone is done by the manufacturer, not an independent third party, or any government agency.
• In 2013 the outer ear, called the pinna, was classified by the FCC as an extremity which can be read in this report here.
• With this classification it means that your outer ear can have the same exposure limits as your hands, wrists, and feet.  Which of course can all be higher than your head.
• All of the SAR values pertain to an adult, not a teen or child.  Please keep that in mind.