Risk of infection from heavily contaminated air

GRUNNET, K. and HANSEN, J. C. Risk of infection from heavily contaminated air. Scand. j. work environ. & health 4 (1978) 336-338. In a factory processing shea nuts the dust concentrations were found to be up to 145 mg/m 3 [80 0/0 res pirable (1-5 ,urn)]. Bacterial examination of the dust revealed that under the worst conditions observed a worker might inhale 350,000 bacteria per 8 h. Of these, 3,000 were Ps. aeruginosa and 1,500 Salmonella spp. of nine different types. The possible health effects of these findings are discussed.

Little has been done to investigate the occupational risk of working in bacterially contaminated air. The risk of infection is not restricted to the diseases normally regarded as airborne. Crozier and Woodward (6) demonstrated that it was possible to infect primates with Salmonella typhi through inhalation of a dose 1,000 times less than would be needed for an oral dose.
Studies have been carried out among sewage workers exposed to aerosols emitted from activated sludge tanks and trickling filters and among workers exposed to aerosols emitted during land application with treated sewage (1,4). These outdoor studies have shown that in practice the health hazard associated with biological aerosols is low, even though the data published so far do not establish the magnitude of this risk. Consequently, an extensive prospective study was initiated among sewage workers (5).
The risk of infection from airborne microorganisms may be greater indoors due to less dilution (10). Apart from hospitals, the degree of bacterial air contamination has been registered only in a few workplaces and industries.
Heavy bacterial contamination of air may occur in industries such as paper pulp industries; the production of vegetable oils; meat-, bone-, and fishmeal factories; and other industries processing contaminated material of biological origin (2).
Since such contamination might represent a health hazard, we decided to investigate the bacterial air contamination in an industry processing imported vegetable material in order to evaluate the risk of infection in relation to exposure.

MATERIAL AND METHOD
Dust was collected in two areas with a varying degree of dust contamination. The concentration of dust in the air was calculated after collection on a Sartorius membrane filter (0.8 porn 160 mm) with a Sartorius Gravicon apparatus, which filtered about 30 m 3 of air per hour.
Prior to the first weighing (four decimals) the membrane filters were dried at 150°C for 3 h and cooled in an exciccator containing silica gel; after sampling the filters were again dried and weighed.
The number and size of the particles were measured with a Climet particle counter (CI-252, Climet Instrument Co., Sunnyvale, Calif.), and the amount of respirable dust was calculated.
Dust for the bacteriological examination was collected under aseptic conditions into sterile bottles from shelves at heights of 1.5-1.8 m above the floor on 1 d in two areas. The shelves had been cleaned the day before collection.
Only this mixed sample was thoroughly investigated by the methods described in this report. However Salmonella concentrations in the same size range have been demonstrated on o'ther occasions.
The dust was suspended in dilution water from which decimal dilutions were made. After filtration of 50 ml of either the 1: 10 6 dilution or the 1: 10 7 dilution, the number of colonies was counted on a membrane filter on the following media: teepol agar, plate count agar, King's agar B (11), Slanetz agar, mannitol salt agar (12), and on Brilliant green agar (8).
All the colonies were isolated from one plate of the following media: nutrient agar, mannitol salt agar, brilliant green agar and teepol agar. After pure cultures had been obtained, all isolates were subjected to the following tests [a modified version of the method of Bonde (3) Some strains were also subjected to special tests, such as elevated temperature and tests for urease.
These procedures result in identification at the genus level, in some cases also at the species level.
At the time of the investigation 3 of the 20 workers had diarrhea. Six fecal samples were collected and examined for pathogenic intestinal bacteria by the Danish State Serum Institute (3 samples) and by our laboratory (3 samples).

Dust examination
The concentration of dust during the work hours varied from 1.5 to 10 mg/m 3 in area I and up to 145 mg/m s in area II.
General bacteriological examination gave a total count of 2.4 million per gram of dust, a figure which corresponds to 3,600-24,000 microorganisms per cubic meter for area I and up to 350,000 for area II.
A special examination for fecal contaminants showed a content of 1,700 coliform bacteria, 140 E. coli, 500,000 fecal streptococci, and 790 Salmonella spp. per gram of dust. The sero-fermentative typing of the Salmonella spp. demonstrated the presence of the following types: S.urbana, S.othmarshen, S.johannesburg, S.montevideo, S.tennessee, S.bloemfontein, S.mbandaka, S.oranienburg, and S.schwarzengrund.

Examination of feces
The examination of the feces failed to reveal any pathogenic bacteria.

DISCUSSION AND CONCLUSION
On the basis of our findings of bacterial concentrations and the respiration volume during light work [13 m 3 /8 h (9)], the daily dose of pathogenic and potentially pathogenic organisms can be estimated approximately as: E.coli 20/day, Salmonella 100/day, Ps.aeruginosa 200/day in area I and up to E.coli 260/day, Salmonella 1,500/day, and Ps.aeruginosa 3,OOO/day in area II.
Compared to the normally accepted infective doses of about a million or more, these figures are low (7).
That no pathogens were demonstrated in the feces from the workers with diarrhea could be due to the fact that the workers were recovering at the time of the sampling. However, since no new cases have been reported during the following 14 months, a connection between the inhalation of pathogenic organisms and the diarrheas seems unlikely, and the reported inhalation doses of, e.g., 100-1,500 Salmonella per day probably do not represent a health hazard to workers. However, the collection of dust by filtration (for weight) may give results that differ from those obtained by sedimentation (for bacteriological examinations). A particle must have an aerodynamic diameter of a minimum of 1 /-lm to contain bacteria. The present investigation showed only 5 % of the airborne particles to be below that size; consequently no great difference in bacterial concentration can be expected between dust collected after sedimentation or by filtration.