Disparity about Near-Infared rays' penetration of the body

[DavidReishi]

Asked in 2015 on "Quora," a PhD candidate answered that near-infared wavelengths can penetrate the body more that visible wavelengths, but only by a millimeter or two.

She sums up her answer, "So the question: can NIR light penetrate the human body? The first millimeter, yes, through the chest cavity and back, not a chance."

Additional responders corroborated her answer.

But then I find this, published at least in part by NASA in 2012:

"The depth of near-infrared light penetration into human tissue has been measured spectroscopically. Spectra taken from the wrist flexor muscles in the forearm and muscles in the calf of the leg demonstrate that most of the light photons at wavelengths between 630- 800 nm travel 23 cm through the surface tissue and muscle between input and exit at the photon detector. The light is absorbed by mitochondria where it stimulates energy metabolism in muscle and bone, as well as skin and subcutaneous tissue."

What gives?

 

[phyzguy]

I suspect the original answer was wrong. Here's a simple experiment you can do. Hold the palm of your hand over a flashlight in a dark room. You can see red light traveling all the way through the tissues of your hand, so it is clearly wrong that it only travels a millimeter or 2.