Fungal acne is Malassezia folliculitis: an overgrowth of Malassezia yeast in the hair follicles (Rubenstein and Malerich, 2014). The yeast cannot make its own fatty acids, so it lives on the fats around it. Paste a product's ingredient list and this checker marks the ingredients that fall in the four classes lab studies found it can feed on.
It gives no safe or unsafe verdict, because the research does not support one. What a match means, and what it does not, is set out further down.
Each rule is a class of chemistry, and each one comes from a study you can read. Most of them were done on yeast grown in a dish.
Malassezia has no gene for fatty acid synthase, so it cannot build fatty acids and has to take them from outside. That was found in the genome of M. globosa (Xu and colleagues, 2007) and then in all 14 species (Wu and colleagues, 2015). In the study the range comes from, the yeast grew only when fatty acids from C12 to C24 were added, and every acid tested in that range supported growth except elaidic and nervonic acid (Porro and colleagues, 1976).
M. furfur split the fatty acid esters it was given and grew on the acids released. How well it did depended on the alcohol half of the ester: ethyl esters were split best, then isopropyl, and decyl oleate only a little (Mayser and colleagues, 1995). A later test of ten emulsifiers found every strain grew on PEG-7 glyceryl cocoate, PEG glyceryl stearate and macrogol-50 stearate, while PEG-35 castor oil was used only by M. furfur (Mayser and colleagues, 1997).
Polysorbates 20, 40, 60 and 80 are sold as Tween, and labs use them as a lipid source to tell the species apart (Guého and colleagues, 1996). That is also why this rule is the weakest of the four. In one identification study, M. furfur used all four, while M. globosa and M. restricta, two of the species the authors call clinically important, used none of them as their only lipid (Kaneko and colleagues, 2007).
Plant and animal oils and butters are mostly triglycerides. Malassezia makes lipases that split triglycerides into free fatty acids (Ro and Dawson, 2005), and whole M. globosa cells split both diolein and triolein (DeAngelis and colleagues, 2007). Grown under olive oil, M. sympodialis and the other newer species released large amounts of free fatty acids from it (Mayser and colleagues, 1998).
Caprylic and capric acid, and caprylic/capric triglyceride, sit below the C12 to C24 range. In lab studies medium-chain triglycerides and medium-chain fatty acids suppressed M. furfur and M. sympodialis (Papavassilis and colleagues, 1999), and glyceryl monocaprylate, a C8 ester, had the lowest inhibitory concentration of the esters tested on M. pachydermatis (Koch and colleagues, 2022).
Some online lists start their range at C11. The growth study behind the range starts at C12, and undecylenic acid (C11) released from its esters was antifungal in the lab (Koch and colleagues, 2020), so this checker starts at C12.
We found no peer-reviewed study showing that ferments such as galactomyces feed Malassezia. One small human trial found a Saccharomyces and Lactobacillus ferment complex lowered Malassezia on sensitive scalps over 28 days (Wang and colleagues, 2023). One trial settles nothing, and ferment filtrates differ by product, so the checker applies no rule either way.
Cetyl, stearyl and cetearyl alcohol, and ethers such as laureth and ceteareth, were not tested in any study behind these rules. The checker says so rather than guess.
Checking for pore-clogging ingredients is a different question with a different body of research. The comedogenic ingredient checker answers that one.