From the Ministry of Labour site:

Is there an acceptable temperature range for workplaces, either hot or cold?

In a health care facility or an industrial establishment, such as a factory, store, shop or office, the regulations set a minimum temperature of 18 °C, subject to some exemptions for things like work outdoors or in freezers. The construction projects regulation specifies a minimum of 27 °C for underground change rooms [Section 260 (3) (d)], a maximum of 38 °C for work chambers [Section 384], and where work is done in compressed air, the provision of a medical lock with a minimum of 18 °C [Section 357] and maximum of 27 °C [Section 380]. There are no set minimum or maximum temperatures for other workplaces. Nevertheless, because either extreme heat or cold may be a hazard, temperature is a legitimate issue in determining workplace safety. A particular concern is heat stress.

What can a worker do about unsafe conditions at work?

Health and safety concerns should first be brought to the attention of the employer or supervisor. If nothing is done, it can be taken to the worker’s health and safety representative or Joint Health and Safety Committee. If the situation is not corrected, it can be reported to the nearest office of the Ministry of Labour. Workers also have the right to refuse unsafe work. OHSA Section 43 outlines the procedure that must be followed, and this process should be understood before a refusal is initiated. More information can be obtained from local ministry offices.

How to refuse unsafe work: