About Wallemia sebi
Wallemia sebi is one of the most extreme xerophiles in the fungal world, capable of growing on surfaces that most molds would consider far too dry. It thrives in dusty environments with elevated relative humidity and is one of the most common molds found in household dust, floor dust, carpets, and mattresses. Because it spreads through dust rather than obvious visible growth, visual checks often miss it. DNA-based tests such as ERMI can detect it in dust. It has been linked to hypersensitivity pneumonitis in agricultural and indoor settings (a distinct entity from the classical 'farmer's lung' caused by thermophilic actinomycetes). Its tiny spores (approximately 2–3 micrometers) are small enough to travel deep into the lower airways. Elevated Wallemia sebi on an ERMI is generally associated with long-term high indoor humidity, even where no water damage is visible.
Ecology and water requirements
Wallemia sebi is classified as a primary colonizer with xerophilic adaptations: capable of germinating and growing at water activity as low as Aw 0.69[2], well below what most molds tolerate. Primary colonizers establish first in damp or even mildly humid environments, often before any visible moisture problem is apparent. Elevated Wallemia sebi on an ERMI test is more often associated with long-term high humidity than with a sudden water event[3]. Common contexts include poorly ventilated areas, conditioned-vs-unconditioned boundary zones (attics, crawl spaces, unfinished basements), and dusty environments where ambient relative humidity is sufficient to support growth on settled organic material.
Mycotoxin biology
Wallemia sebi produces a range of secondary metabolites with biological activity. The compounds documented in the literature for this species are listed in the chips below. Mycotoxin production is not uniform across all strains and is influenced by substrate, temperature, and water activity[4]. The same isolate may produce substantially different metabolite profiles on drywall paper versus wood versus settled dust[6]. DNA-based detection via the ERMI panel identifies species presence but does not directly measure mycotoxin presence; detection of a mycotoxin-producing species in household dust does not establish exposure to that mycotoxin at any specific concentration.
Detection in dust versus visual inspection
DNA-based detection of Wallemia sebi in settled household dust (the basis of the ERMI panel[1]) integrates spore material from current and recent growth in the home, capturing exposure history over weeks to months. A positive detection in dust does not necessarily indicate currently-active growth at the time of sampling; conversely, an active growth site can produce relatively low dust spore counts if the spores are not being aerosolized (sealed wall cavities, behind vapor barriers, or under flooring). Interpretation requires considering the dust signal alongside building inspection, occupant symptom history, and the broader pattern of species detected on the panel.
Where it’s typically found
Wallemia sebi is documented on the following substrates and locations:
- Household dust
- carpets
- mattresses
- floor dust
- dried and salted foods
- wallpaper
- house dust mite habitats
Mycotoxins
Compounds documented as produced by Wallemia sebi (presence and quantity vary by chemotype, strain, and substrate):
Mycotoxin production is not uniform across all strains and is influenced by substrate, temperature, and water activity. Detection of a mycotoxin-producing species in household dust does not directly establish exposure to that mycotoxin at any specific concentration.
On your ERMI report
Wallemia sebi is one of the five HERTSMI-2 species, a subset of the 26 EPA water-damage indicator species (sometimes labeled "Group 1" in EPA documentation). On a HERTSMI-2 score, it is weighted heavily, and elevations contribute multiple points to the total. HERTSMI-2 was developed by Dr. Ritchie Shoemaker as a screening tool for assessing whether a water-damaged building is safe to re-enter for individuals previously sickened in such environments[5]. Whether the level detected in any specific home is concerning depends on the broader pattern: which other species are elevated, the water-activity profile of the elevated species, the home's climate context, and (where relevant) the occupant's sensitivity history. Our interpretation report puts Wallemia sebi in that broader pattern.
Related species
References
- Vesper SJ, McKinstry C, Haugland RA, et al. Development of an Environmental Relative Moldiness Index for U.S. homes. J Occup Environ Med. 2007;49(8):829–833.
- Pitt JI, Hocking AD. Fungi and Food Spoilage. 3rd ed. New York: Springer; 2009.
- World Health Organization. WHO Guidelines for Indoor Air Quality: Dampness and Mould. Copenhagen: WHO Regional Office for Europe; 2009.
- Nielsen KF. Mycotoxin production by indoor molds. Fungal Genet Biol. 2003;39(2):103–117.
- Shoemaker RC, Mark L, McMahon S, Thrasher JD, Grimes C. Research Committee Report on Diagnosis and Treatment of Chronic Inflammatory Response Syndrome Caused by Exposure to the Interior Environment of Water-Damaged Buildings. Surviving Mold; 2010.
- Miller JD, McMullin DR. Fungal secondary metabolites as harmful indoor air contaminants: 10 years on. Appl Microbiol Biotechnol. 2014;98(24):9953–9966.