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A small number of molds can produce poisonous substances called mycotoxins, or cancer-causing poisons called aflatoxins. Some molds that do not produce toxins can still cause allergic reactions or respiratory problems <ref name="usda"></ref>. Their effects are cumulative, rather than immediately toxic as in the case of pathogenic bacteria. A small amount of ingested mycotoxin won't be harmful right away, but it will increase the risk of gastric, intestinal, and colon cancer, as well as other health issues <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2137444642950405/?comment_id=2137519622942907&reply_comment_id=2137542839607252&comment_tracking=%7B%22tn%22%3A%22R2%22%7D Dr. Bryan Heit Heit. Milk The Funk Facebook group thread on the health risks of mold. 06/19/2018.]</ref>. As a general rule of thumb, pigmented molds (non-white/grey) are more likely to produce poisonous substances (especially black mold which could be ''Aspergillus''), however, there are exceptions to this guideline <ref name="Dr. Bryan Heit">[http://suigenerisbrewing.blogspot.com/2016/08/fact-or-fiction-can-pathogens-survive.html "Fact or Fiction? Can Pathogens Survive in Beer - Mould Edition." Dr. Bryan Heit of Sui Generis Blog. 08/11/2016. Retrieved 08/11/2016.]</ref>. Some mycotoxins can survive boiling temperatures in wort to varying degrees, so if mold develops during processes such as kettle souring, it is possible for some percentage of mycotoxins to survive the wort boiling process and fermentation (one study showed that 20-30% of two mycotoxins were removed during boiling and fermentation) <ref>[http://www.sciencedirect.com/science/article/pii/S0168160507003984 Stability of mycotoxins during food processing. Lloyd B. Bullerman, Andreia Bianchini. 2007.]</ref>. Only mold that is growing on in contact with the beer itself is considered potentially dangerous; mold growing in airlocks or outside of vessels should not be a cause for concern as long as they do not come into contact with the beer itself. If only a small amount of mold has grown on the beer, then removing it quickly and purging the vessel with CO² to prevent more mold growth will limit the potential for exposure to toxins. However, if a large amount of mold has grown and if it happens to be a mycotoxin-producing species, then there is a higher chance that more toxin could be in the beer <ref name="Dr. Bryan Heit" />.

As mentioned above, identifying the species of mold based on what it looks like with the naked eye is not a reliable way to identify it at the species level, and thus not a reliable way to know if the mold growth is safe. It isAvoiding the potential risk of mold growth can be somewhat eliminated by visually inspecting the growth and determining if it might be mold growth and pellicle or yeast growth. Visual inspection, however, usually easy may not be enough to tell the difference between general identify whether or not mold growth and is completely absent or not on a pellicle or otherwise, especially for vessels such as barrels which may have more exposure to oxygen at the surface and headspace. Molds can have a similar appearance to yeast/bacteria pellicles to the naked eye. Microscopy is the most accurate way to determine if growths are mold vs yeast growthor bacteria (see [https://suigenerisbrewing.com/index.php/2023/10/20/beer-and-mould "Beer and Moulds – More There Than You Think" by Dr. Bryan Heit] for more information). Mold tends Many molds tend to look "fuzzy" or "hairy" and often has green, red, black, white, or grey colors, whereas yeast activity or pellicles tend to have evidence of CO2 production (bubbles) and look "creamy", "powdery", or "chalky" and is never green, red, or black in color (unless there is material in the beer that is discoloring the pellicle, such as fruit). Mold tends to grow in isolated spots on the surface, while yeast pellicles tend to grow throughout the surface. Sometimes the pellicle is not a solid pellicle, but instead just scattered across the surface in a spotty yet more widely distributed manner compared to mold which would grow in a more isolated area. Protein/carbohydrate coagulation can also be misidentified as mold, however, this also never appears "hairy" or "fuzzy" like mold. We recommend taking a picture through a hand lens when posting an image of what might be mold as this will make it easier to identify. Be sure that the color of the mold/pellicle in the photo is accurate to what it looks like with the naked eye (sometimes cameras make white substances appear more pigmented, which can throw off attempts to identify the organism online). Please make an attempt to identify whether the growth is mold or not on your own before posting in the group (these types of posts clog the stream!). See the images below for examples that should help. Mold can be removed and sent to a [https://www.google.com/search?client=firefox-b-1&ei=Ww08W-fWOabX0gK73K6YBg&q=mold+testing+lab&oq=mold+testing+lab&gs_l=psy-ab.3..0l2j0i20i263k1l2j0l6.127585.128971.0.129112.16.14.0.0.0.0.181.1496.0j12.12.0....0...1c.1.64.psy-ab..4.12.1491...0i67k1j0i131k1j35i39k1j0i131i20i264k1j0i20i264k1.0.5QReSMbR0GU mold testing lab] to possibly be identified as toxic or benign.   

The following table shows examples of mold or pellicles from the Milk The Funk Facebook group, and the consensus on whether mold is likely present or not based on the image. Note that it is likely that barrel aged beer with pellicles can also contain at least some mold. It is not possible to determine whether mold is present or not based only on visual inspection; lab analysis is required to determine for sure if mold is present or not <ref>[https://www.mdpi.com/2311-5637/7/3/174 Piraine REA, Leite FPL, Bochman ML. Mixed-Culture Metagenomics of the Microbes Making Sour Beer. Fermentation. 2021; 7(3):174. https://doi.org/10.3390/fermentation7030174.] See also [https://www.facebook.com/groups/MilkTheFunk/posts/7494013197293496/?comment_id=7495644597130356&reply_comment_id=7495757290452420 this MTF post by Dr. Bryan Heit]</ref>.