Actually this question was raised by one of the panel members to whom we were presenting this project. He asked what if someone is asthmatic and cannot have a controlled breathing system?
First of all user don't have to breathe continuously to keep wheechair operational. For instance, user just need to a hard puff once in order to drive chair in forward motion, similarly a soft puff is needed in order to turn left which would be continuous, the wheelchair will keep on turning until user is giving an input of soft puff.
Apart from that we have also installed a kill switch for safety measures. If something unusual happens user can press it to halt the operations.