A mysterious white powder found in an envelope sent to a public school on the Upper East Side was deemed non-toxic, school officials said.

A secretary came into contact with the suspicious substance while opening mail on the second floor at about 10 a.m. at P.S. 290 on East 82nd Street, according to Department of Education spokeswoman Margie Feinberg.

The victim washed her hands and was decontaminated by first responders. Another employee who was inside the mail room was also being evaluated.

All classes on the second floor were moved to the third floor as a precaution earlier today and the school has since resumed normal activity.