The 1978 edition of Garry Thomson's The Museum Environment tells you on p. 125 how to convert parts per million to micrograms per cubic meter. The formula is in a footnote:
M 1 ppm= ------ ug/m3 0.0224
where M is the molecular weight of the pollutant gas.
Timestamp: Sunday, 03-Mar-2013 21:35:42 PST
Retrieved: Saturday, 25-May-2013 06:05:01 GMT