SAN FRANCISCO, Jan. 9 (Xinhua) -- Two major shareholders of Apple Inc. expressed their concerns of iPhone addiction in children in a letter to the company on last Saturday.

Two big investors are Jana Partners LLC and California State Teachers' Retirement System, who combined own about 2 billion U.S. dollars in Apple shares.

They are worried that the iPhone, one of the most popular smartphones worldwide, have fostered a public health crisis that could hurt children.

The investors urged Apple to create ways for parents to restrict children's access to their iPhones.They also want the company to study the negative effects of obsessive usage of iPhone in children.

"There is a growing body of evidence that, for at least some of the most frequent young users, this may be having unintentional negative consequences," said the letter.

The content of the letter was reported by several U.S. medias but the Apple spokesman declined to comment on the letter so far.