(4) For persons employed in any process or work for which impervious aprons are required under column (d) of the table below and for which washable overalls are not required under column (b) of that table, the occupier shall provide such overalls in addition to the aprons if the Inspector for the District has issued and there remains in force a certificate that overalls are in his opinion needed having regard to the conditions under which the process or work is carried on in the factory to which the certificate relates.