Nowadays the ventilation design of enclosed spaces is still based on the number of air renewals per hour. This coarse approach was developed in the past but nowadays is a fast and simple computational exercise to determine where and when the Threshold Limit Value (TLV) is surpassed. A rather superficial Computer Fluid Dynamics (CFD) analysis is able to provide insights that are useful to propose measures to keep the pollutants concentrations at safe levels. As illustrative example, the contaminants' level inside a factory is simulated, both in steady and unsteady state, for risk and health assessment to determine the contaminant levels reached as consequence of the normal operation or in case of a sudden release. The case study is based on an example related to the ventilation of a printing factory for which there are field measurements available in literature. Two pollutants are emitted separately; one continuously during operation and the other only sporadically during cleaning. A good agreement is obtained between the simulations and previous field measurements. This case is an example that a design fulfilling the legislation according to the air renewals tabulated can surpass the threshold limit value in practice. The main conclusion is that the legislation should be based on CFD results instead of general tabulated values, as in some cases the ventilation could be too oversized and in others may result insufficient.