Pen caps now have requirements that they have holes in them to facilitate air passage in case somebody chokes on the cap.

Here are a few links about pen caps

Info from mfg.com about a bid to manufacture pen caps -

http://www.mfgquote.com/profiles/Milling-and-Turning-82513-Berkeley-California-United-States.html

People actually choke on parts of pens -

http://www.amjforensicmedicine.com/pt/re/ajfmp/abstract.00000433-200012000-00013.htm;jsessionid=G2jKg74ChcLJxTnHlr28nPhlyp2yKqHPZQ38XwgvJpqFHC0vthnw!-447275640!-949856144!8091!-1

Medical info on choking - http://www.medem.com/search/article_display.cfm?path=n:&mstr=/ZZZDYWB1JJC.html&soc=SMS%20of%20WI&srch_typ=NAV_SERCH