CHI's QEHS MS certification is valid for three years from the date of issuance.  Surveillance audits are to be done by the certifying body at least once a year, aside from the internal QEHS audits done by our internal auditors.  In our case, we had opted for twice a year surveillance audits. Our last re-certfication audit was conducted from Febuary 21 to 22, 2012.