Without getting too technical:
Infrared pollution is an anomaly that happens because of new sensor's design, coupled with strong ND filters. ND filters cut light by a number of stops. Anywhere from one to 10+ stops of light can be cut using an ND filter. Older ND filters, however,
don't cut certain spectrums of red and infrared light, as normally these are filtered out by the sensor or film stock. The more stops of light you cut, the less overall light gets through. However, new digital sensors are much more sensitive to infrared light, and don't have the same filters as older sensors. So, the older ND filters cut a certain spectrum of light, without cutting parts of the red and infrared spectrum. So, that's why when you start using denser ND filters (ie 0.6+) your light cuts, but your infrared light stays the same, so the red starts to mix with everything else.
NDIR filters are filters that combine an ND filter and a Hot Mirror filter. This allows you use a single filter tray and essentially get the benefit of two filters. If you already had a set of older ND filters, then you could just buy a Hot Mirror filter (which would cut the IR light spectrum) and then use the NDs in front of it.
The good guys at Lemac get a bit more technical about it:
http://www.lemac.com.au/Support1/Guides/UnderstandingIRandFarRedPollution.aspx
