This is interesting. I never before heard of a "tefillin allergy". Sounds like a good excuse a teenager might make up for not putting on tefillin during a rebellious stage, but according to the article this tefillin allergy is real..

From The Jerusalem Post:

A 14-year-old boy who complained of a chronic rash on his left arm and hand has been diagnosed with a “tefillin rash” – caused by the chemical potassium dichromate, which is used to process the black straps of the phylacteries.


An article in the September issue of the Hebrew-language medical journal Harefuah (of the Israel Medical Association) by doctors at Sha’are Zedek Medical Center describes the unusual case.


The boy was described as the youngest person to be recorded with such an allergic reaction to leather tefillin straps. Other victims have been as old as 77 (such a man was diagnosed after 20 years of suffering from allergic contact dermatitis).


Others were devout men, including rabbis, who wear their phylacteries not only to recite the morning prayers but also keep them on all day as an “extra mitzva.”


“Tefillin allergy” is relatively rare in Israel. It is due not to the tefillin themselves, but to the chemicals used to process the leather straps. Almost all cases involve the phylacteries worn on the arm, but there are rarer cases in which those worn around the head and touching the neck also cause rashes there.


The rash appeared in the boy about a year after he first began to don phylacteries for his bar mitzva, wrote Drs. Pinhas Hashkes and Efraim Sagi of the Jerusalem hospital.


In most cases, the inflammatory response appears in the area of the middle finger, around which the straps are wrapped. It is less common on the forearm and much less frequent on the nape of the neck. In some patients, the rash appears in the form of “straps,” but in others, the rash is not under the places on the skin that are in contact with the straps.


According to estimates, about a quarter of the male Jewish population in Israel wears tefillin on a regular basis. But as they are worn for less than an hour at a time, the allergic response does not appear in all users with sensitivity to the chemical.


Some who were diagnosed turned to their rabbis for permission to wear their phylacteries over their sleeves instead of their bare arm, while others place clear cling plastic under the leather straps. In addition, a Bnei Brak shop called Machon Pe’er sells tefillin processed without the offending chemical.