If they get into the eyes then the eyes will water uncontrollably and the hairs may even cause temporary blindness. This, of course usually puts off the would be attacker, as all thoughts of an eight-legged snack tends to be less of a burning desire. The urticating hairs on a tarantula aren't hollow and loaded with toxins, as they sometimes are with other creatures that use urticating hairs for defence. Tarantulas are not the only creature to use this defence mechanism it is also used by Moths and Butterflies, mainly in their caterpillar stage of development. However, a number of them use toxin loaded hollow urticating hairs and therefore can be accused of carrying chemical weapons which by their nature tend to cause more severe and painful reactions.
Total length ca.
Microscope magnification x440. Detail of barbed distal end. Type 1, 1mm long with coarse barbs along its full length and type 2, just 0.2mm long, very ornate but equally as unpleasant. Some tarantula societies also feel, quite rightly, that careless and uneducated handling of tarantulas can only be damaging to our hobby and could ultimately, in the current 'political correctness culture' be seen as a 'good' reason to limit or ban these fascinating creatures. There are remedies that you can use, however I would suggest that if you are concerned, that you contact a suitable doctor that has knowledge of urticating hairs and their effects as occasionally severe reactions do occur and may require unusual treatment.