Aspergillus and Penicillium are two large groups of fungi naturally present globally. They are often discussed together because their microscopically similar spores are difficult to distinguish in standard air quality tests, leading labs to report them as a combined group. Defining a “high level” is challenging due to the lack of federal or universally adopted regulatory standards for mold spore counts in indoor air. Determining a level of concern relies on a comparative analysis of the indoor environment, not a single number.
Identification and Common Sources
These molds consist of a network of filaments called hyphae, which produce vast numbers of microscopic spores for reproduction. The small, light spores remain airborne for long periods and travel easily through ventilation systems. Both genera are highly adaptable, thriving on organic materials in the presence of moisture.
Indoors, they colonize materials like wood, textiles, paper backing on drywall, and household dust. They are often the first molds to appear after a water intrusion event because many species grow with minimal water activity. Common sources of high concentrations include damp basements, crawl spaces, and poorly maintained heating, ventilation, and air conditioning (HVAC) systems.
Methods for Measuring Mold Levels
The concentration of airborne spores is measured by environmental professionals using specialized sampling techniques. Air sampling often involves a spore trap, which uses a calibrated pump to pull a specific volume of air over an adhesive slide. The resulting data is reported as the number of spores per cubic meter of air (spores/m³).
This non-viable method provides a total spore count quickly but groups Aspergillus and Penicillium due to their similar appearance. Surface sampling, such as a tape lift or swab, is used to identify the specific type of mold present on a surface. A limitation of air sampling is that spore levels can fluctuate rapidly depending on air currents, temperature, and activity within the space.
Criteria for Defining Elevated Levels
Since no absolute legal limit exists, industrial hygienists define a “high level” through comparative analysis and contextual evidence. The most important criterion is the indoor-to-outdoor ratio, which requires collecting an air sample outside the building as a control. Indoor Aspergillus/Penicillium levels should ideally be similar to or lower than the outdoor control.
A level is considered elevated if the indoor count significantly exceeds the outdoor count, often defined as a ratio exceeding 3:1 or 5:1. This disproportionate indoor presence strongly suggests an active, hidden source of growth. Another indicator of a problem is the presence of hyphal fragments in an air sample. A high concentration of these fragments is a strong sign of active, aggressive mold growth within the building structure. Some laboratories may flag raw spore counts exceeding 1,500 spores/m³ as suspicious, but this is always interpreted in the context of the outdoor control.
Health Impact of High Exposure
High-level exposure to Aspergillus/Penicillium spores is primarily linked to respiratory and allergic health concerns. The small spores travel deep into the lungs, triggering allergic reactions like sneezing, nasal congestion, and eye irritation. For individuals with pre-existing conditions like asthma, high spore concentrations can exacerbate symptoms, leading to more frequent or severe attacks.
A more severe, though less common, risk is the production of mycotoxins by certain species found in heavily colonized building materials. The most serious health consequence is invasive aspergillosis, a fungal infection that primarily affects individuals with severely weakened immune systems. This condition involves the mold spreading beyond the lungs to other organs and requires immediate medical intervention.