What causes the irritation is warm temperatures and high humidity—the precription for heavy sweating--on long runs or during a marathon.
When the salt from the sweat are combined with nylon running shirts and/or those made out of synthetics, the nipples get rubbed raw.
Also T-shirts or singlets with silk-screen designs cause abrasion as do shirts with seams across the chest.
Cotton T-shirts usually don’t irritate.
Many runners assume this is just one of those things they have to grin and bear but that’s necessarily the case.
Wearing the right shirts certainly helps, but you can also lubricate your nipples before the run with a petroleum jelly.
Women who wear running bras can make certain the straps and cups are soft enough and apply some jelly or powder before the run or race.
Fortunately there is one lifesaver for runners who suffer from this condition: NipGuards. An outer layer of foam creates a cavity around the nipples which works great to prevent the chafing and rubbing which creates the often bloody mess. Even better, regardless of how much you sweat or how long you run, the NipGuards never fall off.
http://www.lamarathon.com/run/Article.7+M596548a79fc.0.html?&cHash=f67fbfbd26&tx_ttnews%5Bpointer%5D=1
http://completerunning.com/archives/2006/09/12/100-beginner-running-tips/