Warning Mask / Warning Mask

People wear masks to protect themselves from air pollution.
However many masks can not be reused because the materials have the
time limits of usage. But people in different environments is hard to
know whether they need to replace the mask.
The Warning Mask is printed with a special material that can
change the color when the harmful substances absorbed to the
limit. More harmful substances absorbed will make the the icon
change to ! reminding people that the mask is expired and cannot be used.