Activated carbon is a pretty amazing material. It's just carbon, the same stuff as in charcoal, diamond, and the carbon black that shows up on the bottom of pans and kettles used over flame, for those who have gas stoves or enjoy camping. At the same time, it's an incredibly important material for purification, because one of the neat things that activated carbon does is trap toxic stuff by adsorption. It doesn't catch everything, but it catches so many different things that it's often used in gas masks when you don't know what toxic gas you might encounter - for a HazMat team, for example, those who aren't using SCBA tanks.
One unfortunate problem with any filtration system is that the filter itself has a limit to how much crap it can capture from the water or air that's passing through - and the filter can't tell you when it's getting full. How do you know when it's time to change your Brita filter? How does a HazMat team know when their gas masks stop working? In a plant situation with large equipment, you can install sensors to monitor for breakthrough, but that's not practical, and sometimes not even possible, on small portable filtration systems.
A team from the University of California San Diego recently published in the journal Advanced Materials a paper on the production of carbon nanofiber photonic crystals. These are a special crystal form of carbon which, once they've captured toxins, change colour. No power required, no special equipment required, no extra weight for HazMat to carry. These crystals can be embedded right in the activated carbon filter, so they see exactly the same level of toxin as the filter itself. If you embed them at varying depths in the filter, you could actually watch the adsorption front as it moved through the filter, and know with certainty when your filter is getting close to breakthrough, and thus when it needs to be changed.