What's dangerous is fine silica powder. Once inhaled, it stays in the lungs, the same way asbestos fibers and (we'll hear about this in a couple years, trust me) carbon nanotubes do.
Foreign substances stuck in blood vessels can't be good. There is a direct link between asbestos/silica dust inhalation and lung cancer, proven by numerous studies.
Sounds likely to be a future contributor to good ol fashioned black lung disease as from coal mine dust just in a new industry perhaps they'll fins a similar effect from kevlar fibers to that of breathing in fiberglass as well.
on another note:
Concrete dust has a similar effect when the dust is breathed in too--everything will kill us even sawdust. Every time you cook food you are releasing vapors which can be considered carcinogenic we are told we cannot smoke in public places in a lot of regions but it is still ok to light up the BBQ or go to burger king where everything is run on propane or walk along the city streets breathing in automotive fumes of passing cars. Doctors offices and hospitals still expose us to limited radiation from lab equipment. Everything is a danger....you know what the Japanese did after we blew up two of their cities? They rebuilt them! Sure you cannot occupy the epicenters of the blast zones for more than a few hours due to concern for accumulated dosage but the near outskirts are considered habitable. Mt St Helens was thought immediately after the eruption to be a zone that wouldn't again be inhabitable for a thousand years but in the first ten years life came back. I bet we could occupy these disastrous regions such as Chernobyl, 3 mile island, and even this site outside Budapest again sooner than we think.