I am not aware of it causing skin irritation but people can react to the sap of a lot of plants. It is viewed as an invasive weed and if found in your garden you are encouraged to eradicate it.

This is Garden Organics advice on doing so:

http://www.gardenorganic.org.uk/organicweeds/weed_information/weed.php?id=96

This plant has become a pest to our river ways and canals etc, and it would be best to pull it all out before it throws its seed, saying this, to watch the seeds fly is simply amazing, I have never seen anything like it ,quite remarkable, however get rid.

if you have a rash see your doctor