Indoor air pollution due to mycoflora causing acute lower respiratory infections


Chapter 7

Indoor air pollution due to mycoflora causing acute lower respiratory infections



T. Dubey    TBON-LAB, Investment Blvd. Hayward, CA, United States


Abstract


Fungal genera are well known to produce allergens. Several methods have been used to prove that specific asthma and allergy problems in patients are due to the inhalation of specific fungal spores. Due to multitoxic nature of fungal species, physicians are not able to treat a patient with standard methods. Except for a few species of Aspergillus and Penicillium, very limited information is available on the allergens produced by other fungal species and their pathway to cause allergies. Potential risk of a rapid increase in fungal concentrations in the outdoor environment requires a serious monitoring of air mycoflora. In the absence of clinical techniques to confirm mold allergies, preventative measures to avoid exposure and inhalation of mold spores are recommended. An ecological approach to collect fungal data from different locations can help to develop a reliable mold or allergy forecasting system during peak allergy season.



Keywords


asthma

mycotoxin

antigens

ABPA

HP

ecology

sick building spore trap

human activity

standards


1. Introduction


Allergy and asthma are common health issues among humans and they may not get attention until they turn into acute respiratory infections. Mostly respiratory diseases are developed due to continuous exposure to an unhealthy or polluted environment, either indoor or outdoor locations.

Prolonged breathing time in polluted air is mostly possible inside a building, which can be a home or work place (with few exceptions such as agriculture fields and construction zones where field workers are exposed for more than 4 h duration). Normally the preliminary symptoms of allergy are ignored unless they develop into chronic respiratory infections.

Asthma is a common disorder that afflicted the health of 24.6 million persons, including children, in the United States during 2009.1 According to physicians the disease becomes severe when sensitivity toward mold becomes very high.

1.1. Direct association of fungi with development of asthma


Primary reasons suspected for such infections are bacteria and virus, while mold as pioneer invader is still a questionable factor directly associated with such infections. There is a big discussion going on to prove that fungal spores accumulating in breathing air can lead to severe respiratory diseases.

Research published by Knutsen et al.2 has explained the details of fungal species responsible for asthma. They found direct association between increased fungal exposure and loss of asthma control. Jaakkolla et al.3 found that increased sensitivity for Aspergillus and Cladosporium species increased the risk for adult-onset asthma. Harley et al.4 found that children exposed to ascospores and basidiospores during first 3 years of childhood are at risk of asthma. The reason for increased sensitivity for fungi can be a combination of genetic and environmental factors.

Respiratory diseases can be due to outdoor mold spore exposure or due to indoor mold spores accumulated due to a damp environment. Recent reviews from the United States,5 Europe,6 and World Health Organization7 have also confirmed a damp indoor environment as a major factor in the development of respiratory diseases.

Fungal distribution inside buildings may vary depending on the presence of favorable temperature, moisture, and nutritional sources. The presence of mold spores are best represented in house dust collected by HEPA filter. Dust formation takes place due to accumulation of airborne organic and inorganic particulate matter originating from multiple indoor and outdoor sources. House dust consists of a fibrous mixture of hairs, skin cells, dust mites, textile fibers with a particle size range of 10−3 to 1 mm (ibid.).

Besides many other components, the fungi are known to be major microbiota of house dusts. Scotts8 reported from his research on fungi from house dust that outdoor mycoflora act as major supplier of indoor phylloplane spores when wind speed and precipitation favor the release of spores. A large magnitude of fungal spores in house dust can be either amplified within the dust itself or imported from surrounding indoor amplification site or source. The indoor sources include household items varying from small pieces of rotten food from refrigerator, indoor house plant, soil from shoe closets, decaying wood surface, wooden picture frames, or a house pet.911 Table 7.1 shows different environment conditions from residential and industrial/occupational areas responsible for promoting the fungal growth to cause HP and related allergies to exposed persons.


Table 7.1


Various Sources of Allergens Produced by Fungal Species to Cause Hypersensitive Pneumonitis






































Source
Domestic/Residential Exposure Disease Allergen
Excessive moisture: Leakage area, kitchen sink, ceiling, heating ventilation air conditioning filters (HVAC), wall board, shower, basement sewage conditions, Sauna Humidifier air conditioner (from contaminated water), Cephalosporium-hypersensitive pneumonitis (HP), Sauna taker’s lung, hot tub lung Penicillium expansum, Aureobasidium pullulans, Thermophilic actinomycetes, Acremonium (=Cephalosporium), Cladosporium sp.
Spoiled food: Refrigerated food-moldy cheese, rotting fruits, and vegetables Cheese washer’s lung Penicillium casei
Decoration: House plants, live Christmas trees, wooden frames, fish aquariums Asthma, allergies, Housewife’s lung Alternaria, Aspergillus, Cladosporium, Aurebasidium pullulans
Pets: Dogs, cats, and fishes General asthma, Fisherman’s lung Paecilomyces sp., Saprolegnia sp.
Clothes and shoes in closet, sports items/balls Aspergillosis, Sportsman’s allergies

Aspergillus sp.


Smut/Myxomycetes

Outside vegetation: trees, mushrooms, fruits and vegetables, soil Lycoperdonosis Lycoperdon sp./Puffballs, Aspergillus sp., Penicillium sp.
Travel: Items inside car, parks/garden soil: bird droppings, heavy dust Japanese Summer house-HP, Valley fever Trichosporon sp., Coccidioides imitis































Occupational/Industrial Exposure Disease Allergen
Agricultural: Compost, field equipments, vehicles, storage of grain, hay and silage, and sugarcane workers Farmers lung, compost lung, Begassosis Aspergillus fumigatus, thermophilic fungi, actinomycetes, and other soil microorganisms
Mushroom workers Mushroom worker’s lung Micropolyspora faeni is a bacteria associated with mushrooms
Construction workers lung-dust-burrowing animals Valley fever disease Valley fever fungus and other soil fungi
Moldy barley and moldy grapes Malt and wine worker’s lung Aspergillus fumigatus, Aspergillus clavatus, Botrytis cinera
Tobacco workers:moldy tobacco Tobacco worker’s disease Aspergillus sp.
Wood workers: Cedar, mahogany and oak dust and pine and spruce pulp Wood worker’s lung Alternaria species and wood dust fungi

1.2. Characterization of fungi


Fungi are defined as eukaryotic, filamentous, and mostly spore bearing organisms. They grow on plants and animals as saprophytic or parasitic micro- or macroorganisms. Fungal spores vary in morphology and are ubiquitous. Approximately one million fungal species are present in the environment.

Many fungal species are associated with human diseases such as asthma, allergies, respiratory diseases, and skin infections. Respiratory allergy can vary from 6% among general population to 30% among atopic individuals. Toxic metabolites produced by fungal cells can cause allergic manifestations such as asthma, rhinitis, allergic bronchopulmonary mycoses, hypersensitive pneumonitis (HP), and allergic bronchopulmonary aspergilosis (ABPA).

1.3. Historical background


Airborne fungal spores were first associated with allergy attacks among patients by Storm Van Leeuwen.12 He suspected that fungi in the home and mattresses of a patient in Holland who suffered from asthma could be causative agents. Similar studies in Germany and the United States have indicated that airborne fungi could cause asthma and hay fever in many patients. An incident of asthma in a patient who was exposed to fungal spores from the bark of maple logs was reported by Towey et al.13

Another case of asthma attack was noted by Cobe14 in a greenhouse worker who inhaled the spores of Cladosporium from a tomato plant.

Gambale et al.15 studied the airborne fungi of Sao Paulo state of Brazil and reported Cladosporium, Dreschlera, Fusarium, Alternaria, Penicillium, Aspergillus, and hyphal fragments as most common fungal genera.

1.4. Diseases caused by molds in humans


More information is required to track the pathway of individual mold spore type from various stages:

1. Inhaled by a person;

2. Production of specific toxin or allergen in specific area of respiratory tract;

3. Major symptoms generated due to specific allergen;

4. Cause the asthma; and finally,

5. Its proper treatment by using appropriate antigen.

This can be difficult task because more than one fungal species are inhaled by the human body, which may be producing similar toxins but may not have similar antigen for treatment.

Due to this state of uncertainty, we need to look for other options such as prevention of exposure of sensitive persons to toxic mold. This can be possible if we know more about the etiology of predominating mold genera inside and outside a building. Air monitoring of quality and quantity of specific mold genera in surrounding atmosphere is a major tool to watch and prevent the accumulation of spores inside a building.

2. Association of mold with asthma and allergies-major aspects


This chapter will discuss various aspects of predominating mold species associated with indoor air quality and their role in causing any respiratory problems among humans. Major aspects to explore the association of mold species with asthma and allergies and the mechanism of their development inside human body include the following:

1. Pathophysiology: mechanism of development of disease by producing fungal metabolites;

2. Ecological studies: various factors causing mold growth inside the building;

3. Methods to study the mold and related problems in indoor and outdoor environments; and,

4. Induced outbreak of mold allergies due to human activity (eg, bringing a live Christmas tree from the outside of a nursery to the inside of a residential or commercial building).

2.1. Pathophysiology


2.1.1. Toxic metabolites


The major toxic metabolites produced by different groups of fungi responsible for these allergies include:

1. B-Glucan and Dectin receptor: glucans are major components of fungi and may form 60% of dry weight of fungi;

2. Fungal protease: can damage epithelial tissues to enter mucosal layer;

3. Chitinase: can cause increased IgE levels and increase in asthma;

4. HLA-class II antigen: which is found in patients with ABPA; and,

5. IL4RA and IL 13 polymorphisms: genes containing single nucleotide polymorphism (SNPs), which are associated with severe asthma.

Mycotoxins and volatile organic compounds are produced as a major toxic chemical by fungal cells to cause mycotoxicoses but less is known about their exposure limits to cause the disease. Information available on mycotoxins is insufficient from medical textbooks and can be unrecognized by medical professionals, unless they affect large numbers of people.

In 1999, the World Health Organization16 published a bulletin to explain the chemical nature of various mycotoxins and the etiology of disease due to production of specific mycotoxin by a mold species. This bulletin also explains the identification of mycotoxin producing fungi, their epidemiological, clinical, and histological nature and their role (when available) in outbreaks of mycotoxicoses resulting from exposure to different fungi.

The various groups of mycotoxins as explained by WHO bulletin are given as follows:

1. Aflatoxins: These are produced by different Aspergillus species growing on nuts, cereal, and rice if exposed to water or high humidity. These toxins can be harmful to human health. The two major Aspergillus species that produce aflatoxins are:

a. Aspergillus flavus: which produces only B aflatoxins; and,

b. A. parasiticus: which produces both B and G aflatoxins. Aflatoxins [M.sub.1] and [M.sub.2] are oxidative metabolic products of aflatoxins [B.sub.1] and [B.sub.2] produced by animals following ingestion, and so appear in milk (both animal and human), urine, and feces. Aflatoxicol is a reductive metabolite of aflatoxin [B.sub.1].

Aflatoxins are acutely toxic, immunosuppressive, mutagenic, teratogenic, and carcinogenic compounds. The main target organ for toxicity and carcinogenicity is the liver. The evaluation of epidemiological and laboratory results carried out in 1987 by the International Agency for Research on Cancer (IARC) provided evidence that naturally occurring mixtures of aflatoxins can be carcinogenic in humans. Therefore, they are classified as Group 1 carcinogens, except for aflatoxin [M.sub.1], which is possibly carcinogenic to humans (Group 2B).

2. Ochratoxins: These are present as secondary metabolites of Aspergillus and Penicillium strains, found on cereals, coffee, bread, and meat and food from animal origin. The most frequent is ochratoxin A, which is also the most toxic. It has been shown to be nephrotoxic, immunosuppressive, carcinogenic, and teratogenic in all experimental animals tested so far.

3. Trichothecenes: Currently, 148 Trichothecenes have been isolated but only a few have been found to contaminate food and feed. They are produced mostly by members of the Fusarium genus, although other genera [eg, Trichoderma, Trichothecium, Myrothecium, and Stachybotrys (black mold)] are also known to produce these compounds. Deoxynivalenol (DON), also known as vomitoxin, nivalenol (NIV), diacetoxyscirpenol (DAS) are most frequently observed contaminants. On the other hand T-2 toxin is rare.

In experimental animals, trichothecenes are 40 times more toxic when inhaled than when given orally. In several cases, trichothecene mycotoxicosis was caused by a single ingestion of bread containing toxic flour or rice.

Trichothecenes are also very common in air samples collected during the drying and milling process on farms, in the ventilation systems of private houses and office buildings. Trichothecens may accumulate in residential buildings if the walls have high humidity due to seepage. A higher concentration of mold spores can increase the concentration of Trichothecens and can change a healthy green building into a sick moldy building. This can be proven when the buildings and ventilation systems are thoroughly cleaned and symptoms of airborne toxicosis disappear.

4. Zearalenone: Earlier known as F-2, this is produced mainly by Fusarium graminearum and related species, principally in wheat and maize. Zearalenone and its derivatives produce estrogenic effects in various animal species and cause infertility, vulval oedema, vaginal prolapse, mammary hypertrophy in females, and atrophy of testes and enlargement of mammary glands in males.

5. Fumonisins: These are mycotoxins produced by Fusarium moniliforme and related species when they grow in maize. Only Fumonisins B.sub.1and B.sub.2 are of toxicological significance. Disaster due to this mycotoxin Fumonisin [B.sub.1] was best observed during the outbreak of acute food-borne disease in 27 villages of India. The individuals affected were from the poorest social strata, who had consumed maize and sorghum harvested and left in the fields during unseasonable rains. The main features of the disease were transient abdominal pain, borborygmus, and diarrhea, which began half an hour to 1 h following consumption of unleavened bread prepared from moldy sorghum or moldy maize.

2.1.2. TTC concept


The threshold of toxicologic concern (TTC) is based on a known structure-activity relationship. Frawley17 and Munro18 emphasized methods of evaluation of safety and characterization of risk to determine the daily chemical intake level that is acceptable for the human body without causing adverse health effects. This was assumed for a 70 year life span (Hardin et al.19). The TTC concept was further extended to other inhaled substances by converting a TTC expressed in microgram/person/day to an airborne concentration (ng/M3). The resulting concentration of no toxicologic concern (CONTC) (30 ng/M3) is considered as a generic airborne concentration that is harmless to human. It was also suggested that mycotoxin levels of dust and fungal spores from agricultural fields have potential to produce mycotoxins greater than CONTC levels. On the other hand, common exposure to mycotoxins inside buildings are below CONTC and harmless.

Unacceptable relationship of airborne mold exposure and clinical manifestations of allergic rhinitis. Bush et al.20 evaluated various data to establish a clear relationship between mold and respiratory diseases and gave following reasons for the unclear relationship between the mold as causal organism and human respiratory diseases.

1. Presence of IgE antibodies to molds as part of polysensitization among atopic patients (those with allergic asthma, allergic rhinitis, and atopic dermatitis).

2. Lower airway diseases (asthma) have well known allergic responses to inhaled mold antigens.

3. There is no evidence to prove that exposure to outdoor airborne molds can cause allergic rhinitis. In addition, the studies on the contribution of indoor molds to upper airway allergy are even less compelling.

4. Exposure to airborne molds is not sufficient to prove them to be a factor in atopic dermatitis, urticaria, angioedema, or anaphylaxis.

It was also suggested that patients with suspected mold allergy should be evaluated by means of a standard clinical method for evaluation of potential allergies (skin or blood testing) for IgE antibodies in response to appropriate mold antigens.

2.1.3. Role of prick test


According to Menezes et al.,21 the skin test is a deliberate and controlled exposure to a suspected allergen conducted mainly to confirm clinical atopic sensitivity. The skin test has recently become a widespread diagnostic aid. A basic premise of skin test is that reaginic antibody will fix to the skin mast cell similar to that occurring in other target organs. It involves an interaction between the allergen and IgE fixed to mast cells in the skin with liberation of chemical mediators, which results in local erythema and wheal formation.

In a clinical review by the European Respiratory Society, Kousha et al.22 emphasized the fact that air born fungi are spread through atmospheric air and if we know the ecological distribution of anemophilous fungi in a city then specific treatment of allergic manifestations induced by inhaled allergens can be easier. Their use in an individual’s allergy is widespread. The authors used the prick test in 50 patients suffering from asthma and rhinitis and 10 healthy persons with no resp. allergy as control. Fungal allergens extracts were prepared from 10 of the most predominant mold species found in air by using sodium bicarbonate. All ten of the predominant fungi could provoke skin test reactivity in individuals with resp. allergy in a specific geographic location. Aspergillus, Alternaria, and Dreschlera were positive in all 50 patients. Against this, all of the healthy people used as control tests were negative.

2.1.4. Allergic bronchopulmonary aspergillosis


Allergic bronchopulmonary aspergillosis (ABPA) is a hypersensitivity reaction to Aspergillus fumigatus antigens. This is generally seen in patients with atopy, asthma, or cystic fibrosis. Kousha et al. have also reported that invasive pulmonary aspergillosis occurs primarily in severely immunodeficient patients and that the number of such patients have increased.

The effect of inhalation of A. fumigatus spores to different types of sensitive patients include:

1. Normal host-no sequel;

2. Patients with cavity lung disease: aspergilloma;

3. Chronic lung disease or mild immune-compromised host: chronic necrotizing aspergillosis;

4. Immunocompromised host: invasive pulmonary aspergillosis; and,

5. Patients with asthma, cystic fibrosis, and atopy can lead to allergic reaction.

2.1.5. Sensitization to ABPA


Sensitization to Aspergillus antigens is an important phenomenon in asthmatics, especially those with atopy but this also increases the risk of more severe airflow obstruction and more prescriptions for oral corticosteroids. These observations also suggest that it is crucial to screen asthmatic patients for sensitization to Aspergillus antigens and to monitor these patients more closely and exclude the presence of ABPA.

ABPA is usually suspected on clinical grounds. The diagnosis is confirmed by radiological and serological testing. Other symptoms include elevated IgE total serum and sputum cultures reveal Aspergillus spp. Elevated Serum IgE could be used as a marker for flare-ups and response to therapy.23

The pathogenesis of ABPA is not completely understood. There is no clear correlation between Aspergillus load in the environment and the development of ABPA.24 More than one immune responses may be involved,25 such as Aspergillus-specific IgE-mediated type I hypersensitivity reactions5, specific IgG-mediated type III hypersensitivity reactions,6 and abnormal T-lymphocyte responses.7

2.2. Ecological studies


Direct association between mold species and human disease caused due to inhalation of mold spores is a big challenge among medical mycologists. One of the major reasons behind this fact is lack of sufficient data to create a standard for mold concentration inside the buildings.

Various efforts are being made to furnish more reliable data from different habitats to prove that fungal species can be one of the strongest initiators in causing the upper and lower respiratory diseases of the human body.

Current research data suggest the following:

Various mold spores present in outdoor air can act as source of indoor mold exposure;

The occurrence of various mold species in different geographical regions play a major role in outbreak of allergy season of the area;

Different mold genera are associated with specific substrates inside a building and serve as source of allergies to sensitive persons;

High moisture level supports mold growth and plays a significant role in converting a healthy green building into a moldy and sick building; and,

Mold spore concentration collected from indoor environment must be compared with same from outdoor environment and remediation should be planned accordingly.

In the following section we describe the major ecological studies on mold spores associated with indoor air quality.

2.2.1. Studies by James Scott


James Scott in his thesis8 studied the mycoflora from indoor household dust. According to his observations, the development of allergies to fungi follow the biological process similar to other allergens such as pollen, dust, dandruff and so on. The airborne spores of fungal species such as Alternaria, Aspergillus, and Cladosporium are found throughout the world and can cause allergic rhinitis and allergic asthma. Fungal spores are especially important in the tropics where climatic conditions are favorable for growth and sporulation. Higher concentrations of spores can cause an increased incidence of allergic diseases. He isolated Aspergillus and Penicillium as predominant fungal genera collected as vacuum dust. These species were reported as major sources of asthma and allergies to the exposed persons. He also used molecular PCR techniques to identify different species of Penicillium collected from house dusts. These fungal species can be present on different surfaces, including carpet, and were collected as house dust where only small-sized spores with longer viability can survive to accumulate and cause respiratory diseases.

Scott also studied the characterization of fungal diversity of house dust and found major predominating fungal genera as Aspergillus, Alternaria, Chaetomium, Fusarium, Myrothesium, Paecilomyces, Penicillium, Stachybotrys, and Tolypocladium. He also emphasized analysis of household dust as a major storehouse for dominant indoor fungal taxa such as Penicillium and its role in causing the human respiratory diseases.

Aspergillus and Penicillium types are accumulated in house dust because they are small in size (2–5 um) and can easily penetrate the dust bags of vacuum cleaner to regenerate and accumulate in carpet dust.

There were noted correlations between positively associated dust borne fungal species and their ecological similarities, as described by Pope and coworkers.26 They reviewed asthma trends and observed that magnitude of allergen exposure increased the potential for allergic sensitization, and suggested an increase in asthma morbidity and mortality as reflected by hospital admission and statistics.

Many of these molds are passive and allochthonous arising from different outdoor habitats. These mold genera also form specific assemblage of species from dust microform of many ecologically homogenous groups,8 as shown:

1. Phylloplane (leaf surface) molds: Allernaria alternata, Cladosporium cladosporioides, Epicoccum, Aspergillus versicolor;

2. Xerophilic molds: Eurotium herbarum, Wallemia saba. They are present due to activities within indoor environment, such as food and clothes;27,28

3. Soil borne fungi, present due to indoor movement through footwear, transferred by pets or by potted ornamental plants29: Trichoderma viride, Penicillium citrienigrun30,31;

4. Assemblage of fungal species as contaminants of water damaged building material: A. versicolor, Aspergillus ustus, Chaetomium globosum, Penicillium aurentiogriseum, Penicillium brevicompactum, Penicillium chrysogenum, and Stachybotrys chartarum.

Accumulation of small-spored fungi also found a disproportionate high levels of Aspergillus and Penicillium in household dust which could be due to morphology, long viability, thin wall, and globose shape.32,33

2.2.2. Studies by Dubey


Dubey and Amal34 noted that many household items from daily use can provide the substratum for mold growth and accumulation of spores. These materials are used in residential and occupational buildings and can increase the spore concentrations and cause allergies to sensitive persons. They focused on the colonization of indoor household items by Aspergillus species and used viable bulk culture method. An affinity of Aspergillus species with 14 different household substrates was noted, which included fiber, wall material, wall paper, wooden material, leather, textiles, and so on. CFU counts of different species of Aspergillus were recorded from 374 samples collected from different buildings of Northern California during 2003. A. versicolor was most predominant species followed by Aspergillus niger, A. fumigatus, and A. flavus. Wall material, wall paper, and fiber/textile were preferred by 80% of species. More allergic mold from wall material also suggested a need for thorough inspection of wall materials while looking for the source of mold growth.

A summary of various items serving as possible mold spore source from residential and occupational environments is presented in Table 7.1.8,9,11,3438

Dubey39 collected data on ecology and distribution of mold spores from 18 different locations of Northern California. This was a preliminary survey on distribution of mold genera from outdoor environments of 18 different cities of Northern California (Table 7.2).


Table 7.2


Distribution of Predominating Mold Species From 18 Different Cities of Northern California

















































































































































































































































Date City Ascospores Aspergillus/Penicillium Type Basidiospores Cladosporium Total Counts
Spores/M3 Spores/M3 Spores/M3 Spores/M3 Spores/M3
May 14 Alameda
800 1,750 1,000 5,150
Aug. 14 Brisbane 1,650
1,750 900 5,800
Jun. 14 Concord
1,250 700 1,300 4,050
Aug. 15 Concord 500
1,000 1,650 3,750
May 15 Crescent City 4,300 2,000 8,750
16,550
Apr. 15 Danville

2,100 2,250 5,700
Apr. 14 Dublin

1,250 1,000 3,400
Jul. 15 Dublin

1,200 1,300 3,250
Aug. 15 Eureka
2,500
1,250 5,500
Jul. 15 Eureka
1,500
1,750 3,700
May 15 Eureka
4,750
500 6,200
Feb. 14 Fremont 2,150
2,200 1,200 6,850
Mar. 15 Fremont 6,000
10,000 1,250 21,000
Jul. 15 Hayward
2,000 750 1,750 5,350
May 15 Hayward 1,100
1,200 1,700 4,700
Aug. 14 Hayward 7,640
4,770 2,640 20,500
Mar. 15 Las Gatos
1,000
3,200 7,400
Apr. 15 Livermore 1,000
2,200 1,500 4,400
Aug. 10 Livermore 1,000 1,150
1,250 4,500
May 15 Napa 1,200

6,400 9,350
Aug. 14 Oakland 750
1,250 1,100 4,150
Jun. 15 Oakland
2,250 2,000 1,700 7,500
Apr. 15 Oakland

3,000 1,500 6,300
May 14 Petaluma
1,250 3,000 1,750 7,700
Jun. 14 Pleasanton
1,350 3,350 850 6,600
Jul. 15 Redwood City
2,000
1,500 6,400
Aug. 15 San Francisco 300
1,650 1,300 3,650
Jun. 15 San Francisco
3,250 680 550 4,650
Jun. 15 San Francisco
700 1,250 1,750 4,850
Feb. 14 Sunnyvale
500 1,000 1,350 3,500
Total Observations 30 12 16 23 29

% Occurrence 40 53.3 76.6 96.6

% Dominance 0.08 16.6 52.1 37.9

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