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Urban Airborne and Settled Dust | Smuts, Periconia, Myxomycetes

Urban Airborne and Settled Dust

By Dr. John Shane

Imagine a thick, whitish haze over 1,000 miles long, moving slowly from Mongolia or the Sahara Desert to distant places like Miami, Denver, or Boston. Every year millions of tons of dust are deposited in places all over the United States from distant sources. These dust clouds bring with them fungi, insects, pollen and all kinds of non-biological particles. These dusts have been implicated in causing problems like the sea fan coral decline on the coast of Florida and numerous human health effects. Distant dust can amount to over 25 micrograms per cubic meter of air.

Most of this distant dust is ultrafine (below 2.5 μm) and is not seen or identified in routine IAQ analysis. The EPA regulates fine dusts (PM 2.5 and PM 10 regulations). The relevance of dusts to the IAQ industry is usually confined to questions like, "Why is my house so dirty?" or "What is that black stuff on my carpets and walls?", or even "Is this dust pitting the paint on my car?". All of these questions can be answered by the trained microscopist.

Indoor Air Quality is part of industrial hygiene and is concerned with the study of hazards to human health. Microscopic particles in the air are one of these hazards and, for the last several years, a lot of emphasis has been placed on fungal spores. Our emphasis here is to concentrate on the dust being generated by everyday activities close to or in our homes. Dusts in commercial buildings can be highly specific and is the subject for a later article.

In modern, energy efficient homes, dust has a harder time getting out than in. Recirculating kitchen fans that do not exhaust to the outside, better insulation, self-contained chimneys on furnaces that do not allow mixing with the outside air, better lighting, more synthetic materials that shed particles and closed windows are all part of the problem. As a consequence, homeowners are finding that their newer homes are showing excessive dirt. You have probably noticed in your own neighborhood or inside your house that dust settles constantly. These settled dusts are mostly from local sources like your carpet, clothing, skin and the roads outside. Road dust in urban areas accounts for most of the dust seen in outside air samples.

In addition to pollen and fungal spores, particles captured on indoor air samples include plant hairs, carpet fibers, cotton, degraded feathers, insect debris, starch grains, glass fibers, skin cells, plant fibers, printer ink, and oil droplets. The list goes on and on. The sampling method largely determines which and how many of these particles them you will find.

The most noticeable and easy to identify household debris captured on air samples are starch grains, cotton fibers, oil droplets, and plant debris. Present to a lesser degree, and certainly harder to see and identify, are particles derived from erosion and combustion. Most of these particles are derived from inside the home; but, new particles of interest are emerging and most of these are from non-biological sources.

Should the field investigator be concerned about debris loads in their samples? To answer this question one has to know what kind of debris is being captured and what can you do about the source. Of course, this is at least part of the point of a building inspection and surface and air sampling. The full answer depends on what the client wants, but certainly particle identification and particle source generation can tell you something about the history, use and health of a building.

What kind of sampler(s) are needed to look for particles other than pollen and spores? Fortunately, some of the current air samplers on the market today offer adequate efficiency for sampling many of the particles of interest to the homeowner including the Allergenco, Burkard, and Zefon Air-O-Cell.

Here are a few examples of how particle identification has helped solve IAQ investigations.

Two feuding neighbors had accused each other of doing all sorts of unkind deeds to each other's property over a span of several weeks. One of the neighbors noticed that their car in their garage was covered in a sticky substance and accused their neighbor of doing it. A routine swab of the car revealed the sticky substance to be nearly 100% insect borne pollen. The sticky slime was a result of mass bee defecation. This was a natural event that the other neighbor would have had difficulty coordinating.

Another example involves a problem encountered during a proof of remediation sampling. Air samples had been taken in a building in which mold remediation had been performed. The samples, intended to "clear" the building, showed a large number of small round, dark spore-like particles. These spheres turned out to be welding spherules that were being spread throughout the building from duct fabrication that was occurring in the basement. A correct identification of these spore-like particles saved time and money by identifying the particle. No need to find the hidden mold source that was initially suspected.

Particle identification adds another dimension and investigative tool to the pollen and spore counts already being provided by labs. In many cases, the samplers can be the same and the time required to analyze the particles does not greatly slow down the reporting time of the samples.


Fungus of the Month: Smuts, Periconia, Myxomycetes

By Dr. Srivandana Kilambi

This a grouping of three types of spores broadly resembling one another in size, shape, color and surface ornamentation. While it is hard to distinguish between one and the other in a spore trap, on a tape lift Periconia spores, with its underlying sporulating structures and myxomycetes, with its lacy fruiting bodies can be identified.

They are commonly recovered in the outside air with recovery rates between 55% in January and 85% in June. The levels recovered are fairly low, with the 50th percentile value, when recovered, ranging from 27 spores per meter cubed in January to 80 in June. See Figure 1.

Frequency of detection and spore densityof smuts, Periconia, and myxomycetes by month

Figure 1: Frequency of detection and spore density of smuts, Periconia, and myxomycetes by month. The gray bars represent the frequency of detection, from 0 to 1 (1=100%), graphed against the left axis. The red, green, and purple lines represent the 2.5, 50, and 97.5 percentile airborne spore densities, when recovered, graphed against the right hand axis. (Source: EMLab™ MoldRANGE™ data. Total sample size used for this graph: 39,878.)

Their distribution across the U.S. varies widely, for example, it is recovered in the air 30% of the time in Virginia and 80% of the time in Texas. It is still only present in low number with one of the highest 50th percentile concentration values, when recovered, only being 133 spores per cubic meter of air (in South Dakota).

Smut Fungi:
Smut means a particle of dirt or a smudge made by soot smoke or dirt. True to its literary meaning, the name smut fungi refers to the black, dusty masses of teliospores (resting spores or overwintering spores) resembling soot or smut that form in diseased plants. All smut fungi are biotrophic (plant pathogenic) in nature with shorter or longer saprobic (living on dead organic matter) phases. They belong to the class basidiomycota and order Ustilaginales. More than 1,200 species of smut fungi in more than 50 genera attack approximately 4,000 flowering plants belonging to over 75 families - most commonly the grass family (Gramineae) and the sedge family (Cyperaceae), causing various kinds of interesting symptoms. Some smut fungi are confined geographically to smaller areas, while others are found wherever their hosts grow. Since smut fungi grow on plants and do not generally colonize wet building materials, they are almost exclusively of outdoor origin but can brought indoors through the normal influx of air currents, on people's clothing, etc. The spores are dry and dispersed through splash from rain or blown by heavy winds. This may seem counterintuitive; but, similar to Penicillium and Aspergillus, the spores are hydrophobic so they don't really get wet and the rain acts as a mechanical disturbance only.

The life cycle of smut fungi has two distinct phases - dikaryotic phase (each hyphal cells contains two nuclei - one of each mating compatibility group) and homokaryotic phase (yeast like - having genetically identical nuclei). In nature, the dikaryotic phase or teliospores appear to be obligately parasitic on flowering plants while the homokaryons are non-pathogenic. The teliospores arise from the dikaryotic hyphae that grow in the intercellular spaces between the host plant cells without causing any sign of disease. The hyphae are slender, septate, and often coiled structures. Along with the plant, the dikaryotic hyphae also start to grow and the two nuclei of each cell combine to form diploid cells and the walls thicken to produce the mature spores. These teliospores are characteristic resting spores of the smut fungi that germinate in different ways giving rise to yeast like basidiospores.

Macroscopically most smut spores in mass, appear yellowish, brownish or blackish in color. However, when viewed under a microscope, spores appear as dark reddish or black. Most of the teliospores are globose in shape and have surface ornamentation (with spines, warts or ridges), but some appear smooth. In many species the spores are solitary but in some cases they tend to cement together and form specialized spore balls. Teliospores of some smut fungi are capable of surviving in soil for many years. Germination of these can be affected by many factors such as temperature, humidity, PH, light, and presence or absence of different substances. Smut spores are thought to be allergenic by many, though research is needed to determine the details.

Economically smuts are very important, causing millions of dollars worth of damage to important crops as well as ornamentals. Some of the common smut fungi species that infect different food crops are: Ustilago maydis - common corn smut; U. avenae - loose smut of oats; Urocystis cepulae - onion smut; and species of Tilletia - bunt diseases of various grains.

Smutted grains give off the offensive odor of trimethylamine that causes the grain to smell like rotting fish and they are potentially combustible in storage and loading facilities. Teliospores that accumulate in machinery while harvesting and handling can explode violently causing damage to personnel and the machinery. However, the galls produced on infected ears of corn by Ustilago maydis are edible and used as food in parts of Mexico. In recent years a commercial market for these galls known as "cuitlacoche" or "huitlacoche" or maize mushrooms also developed in United States. There are some interesting recipes on internet if you are curious enough or brave enough to try.

Periconia Species:
Periconia is a dematiaceous (dark pigmented) fungus, first described by Tode ex Fries in 1791. It is a wide spread outdoor fungus commonly found on various substrates including stalks of grasses, herbaceous stems, dead leaves or leaf spots. These are also common symbiotic root colonizing fungi in the tall grass prairie. Almost always associated with other fungi in nature. Approximately 20 species have been identified. They form dry spores that are wind dispersed. Periconia species are rarely found amplifying indoors on building materials; however, spores do find their ways indoors from outdoors. It is not known whether the infrequency of occurrence of this fungus is due to the types of substrate found indoors, its specific growth regime, or its poor competitive abilities with other more common fungi like Aspergillus or Penicillium.

Colonies on a media appear small and compact, grey, brown, olivaceous brown or black. Conidia (spores) of Periconia are pale to dark brown in color, spherical in shape and having surface ornamentation (spiny or warty). Spore size ranges from 16 to 18 microns in diameter. Spores occur in chains and these chains are often branched, arising at one or more points on the curved surface of conidia producing cell.

Popularly called slime molds, these are not considered true fungi genetically. They were once considered to be animals due to their creeping phase. Anton de Bary, one of the founders of mycology and the first myxomycologist called these organisms Mycetozoa (Greek: myketes-fungi and zoon-animals) in 1887 and considered to be more closely related to protozoan than the fungi. In 1833 Link did work on these organisms and considered them as fungi. Now, mycologists consider these strange organisms to belong to a class called Myxomycetes (myxa-slime and myketes-fungi), which was first used by Macbride in his 1899 monograph of the slime molds. Even, today the true relationship of myxomycetes to fungi remains unclear. Myxomycetes are cosmopolitan in nature, but a few appear to be confined to the tropics or subtropics and some only to temperate regions. They normally grow in cool, shady, moist places on decaying wood, leaves or other organic matter retaining lots of moisture. Bark mulch in a flower garden, shrub bed or over watered lawn are a few examples. Slime mold feed on decaying organic matter, bacteria, protozoa and other minute organism, which it engulfs and digests. In the terrestrial ecosystems of temperate regions, myxomycetes are associated with a number of microhabitats like coarse woody debris on the forest floor, the bark surface of the living trees and the forest floor leaf litter. Each of these microhabitats tends to be characterized by a distinct assemblage of myxomycetes. Approximately 1,000 recognized species of myxomycetes are known.

The colonies of slime mold living on logs and bark mulch can be colorful in yellow, orange or red. Some produce cream-colored masses of cells along grass blades. Slime molds often appear in the same area of the lawn year after year in a four or six inch patches in various shades of purple, gray, white or cream. Spores are round, mostly ornamented, reticulate (covered by a network of ridge), echinate (spiny), verrucose (warted) or asperulate (finely warted). Spores can be classified as either dark (black, violet, brown & purplish brown) or light or brightly colored (red, yellow, orange, white, pale gray, pink, light or rusty brown). Spore size ranges from 5 to 15 microns in diameter. Myxomycete spores are considered to cause Type 1 allergies (hay fever and asthma).

1) Alexopoulos, C.J., Mims, C.W. & Blackwell, M. Introductory Mycology. 1996. 4th ed., John Wiley & Sons, Inc., USA.
2) Ellis, M.B., 1971. Dematiaceous Hyphomycetes. Common Wealth Mycological Institute, Kew, Surrey.
3) PubMed


This article was originally published on October 2005.