Mesophilic

Alternaria alternata

Alternaria alternata is the outdoor mold most strongly associated with allergic asthma, and it has been implicated in thunderstorm asthma events.

Environmental referenceMesophilic · Aw ≥ 0.85

Moisture niche · where this species can grow

XerophilicAw < 0.80
MesophilicAw 0.80–0.89
HydrophilicAw ≥ 0.90
Min Aw 0.85
← Drier conditions toleratedRequires liquid water →
Minimum Aw
0.85
EPA reference (GM)
35 SE/mg
Mycotoxin profile
4+ documented
Ecological role
Secondary colonizer

About Alternaria alternata

Alternaria alternata is the outdoor mold most strongly associated with allergic asthma, and it has been implicated in thunderstorm asthma events. As a Group 2 mold, some Alternaria is expected indoors as a reflection of outdoor spore levels. Much higher indoor levels can point to an indoor moisture source. Its allergens (Alt a 1, Alt a 3) are cross-reactive with other mold and plant allergens.

Ecology and water requirements

Alternaria alternata is classified as a secondary colonizer with mesophilic moisture requirements: capable of growing at water activity in the range Aw 0.85 and above[2], typical of condensation surfaces and freshly damp materials. Secondary colonizers follow primary xerophilic species in the ecological succession that develops on damp building materials. Detection of elevated Alternaria alternata on an ERMI suggests intermittent or recent moisture conditions consistent with condensation, vapor permeation, or short-lived water events: chronically cool surfaces (uninsulated walls, single-pane windows, plumbing chases) and bathrooms or kitchens with insufficient ventilation are typical contexts.

Mycotoxin biology

Alternaria alternata 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[3]. The same isolate may produce substantially different metabolite profiles on drywall paper versus wood versus settled dust[4]. 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 Alternaria alternata 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

Alternaria alternata is documented on the following substrates and locations:

  • Window frames
  • shower grout
  • damp textiles
  • decaying indoor plants; primarily from outdoor air infiltration

Mycotoxins

Compounds documented as produced by Alternaria alternata (presence and quantity vary by chemotype, strain, and substrate):

Alternariol· some strainsTenuazonic acidAlternariol monomethyl etherGenerally low concern indoors

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

Alternaria alternata is one of the 10 EPA environmental reference species used to compute the SLG2 component of the ERMI score (sometimes labeled "Group 2" in EPA documentation). Group 2 species are commonly found in outdoor air; their levels are subtracted from the indicator-species sum, which means high outdoor-reference detections actually reduce the final ERMI. A low-level detection of Alternaria alternata is common, because these species are also found in outdoor air. Our interpretation report reads its level alongside the indicator species.

Related species

Aspergillus flavus/oryzaeAw ≥ 0.78 · XerophilicAspergillus fumigatusAw ≥ 0.82 · MesophilicAspergillus sclerotiorumAw ≥ 0.80 · MesophilicAspergillus unguisAw ≥ 0.83 · Mesophilic

References

  1. 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.
  2. Pitt JI, Hocking AD. Fungi and Food Spoilage. 3rd ed. New York: Springer; 2009.
  3. Nielsen KF. Mycotoxin production by indoor molds. Fungal Genet Biol. 2003;39(2):103–117.
  4. Miller JD, McMullin DR. Fungal secondary metabolites as harmful indoor air contaminants: 10 years on. Appl Microbiol Biotechnol. 2014;98(24):9953–9966.