Occupational biohazards include not only factors that have long been known (viruses, bacteria, fungi, parasites), but also agents exerting allergic and toxic effects, which are directly responsible for the development of various diseases in many occupational groups. Numerous agents of this group (allergens, microbial toxins, pollen allergens and allergens of animal origin) are components of bioacrosols--potential hazards inducing occupational respiratory diseases among farmers and people involved in other occupations. Contrary to the majority of chemical and physical factors, neither commonly approved criteria for assessing exposure to biological factors, nor threshold values and methodological recommendations are as yet available. The lack of these criteria renders it difficult to implement in Poland and in other countries Directive 2000/54/EC on the protection of workers against the risk of occupational exposure to biohazards, issued by the European Community. The Institute of Rural Medicine in Lublin has drafted the proposals for threshold limit values of occupational exposure to bioaerosols associated with plant and animal dusts, including: mesophilic bacteria, Gram-negative bacteria, thermophilic actinomycetes, fungi and bacterial endotoxins. These proposals could be considered as a starting point for developing appropriate facultative standards that would facilitate the practical implementation of the aforesaid Directive. Meantime it is essential to be strict in following the binding concentration limits of the plant and animal dusts in the air.