While methods for characterizing microorganisms have improved significantly in recent years, there has been little application to the study of bioaerosols. This is particularly the case in the wastewater treatment industry, where workers can be exposed to a veritable cocktail of airborne pathogens. Researchers wanted to fill this gap.

Sewage in treatment plants releases a variety of pathogens—bacteria, viruses, protozoa—into the air. However, little is known about the exact composition of these bioaerosols. An IRSST-funded team, led by Caroline Duchaine of the Quebec Heart and Lung Institute Research Centre, tackled the problem.

They combed through 10 sewage treatment plants, including two outside Québec, to document the various bioaerosols. Using state-of-the-art molecular techniques, they were able to obtain an accurate picture of the air quality in these facilities.

To monitor air contamination, industrial hygienists usually measure the total culturable bacteria in a given environment. However, these measurements do not tell the whole story, as the researchers discovered.

Generally, the air content reflects what is in the sewage. Many fecal coliforms are found there, including Gram-negative bacteria, which have the capacity to produce endotoxins. Endotoxins can cause respiratory and gastroenteric health problems.

“We realized that even when the quantity of culturable bacteria was below the recommended exposure limit, the level of endotoxins could exceed the limit," notes Ms. Duchaine. This can have a direct impact on the workers. "If an employee decides not to wear a respirator because the bacteria are below the limit value, that person is still exposed to a level of endotoxins higher than the international recommendation," warns Marc Veillette, one of the researchers on the team.

Seasonal Variation

The workers’ personal bacteria levels were also measured. About 15 of them wore personal samplers for two weeks to confirm data previously collected from sampling stations. The researchers also wanted to see if their exposure varied with the seasons.

With Québec's harsh winters, wastewater treatment has to be done indoors. However, there have been very few studies on the effect of seasons on worker exposure. In confined spaces, air quality is generally better in summer than in winter because there is more ventilation. But sewage treatment plants seem to be an exception to the rule.

“We found the bioaerosol levels were higher in summer than in winter. Because the sewage is warmer, there are more bacteria and they reproduce more quickly," Ms. Duchaine explains.

The results of the worker samples are consistent with the data from sampling stations. During the summer, the estimated exposure of three workers even exceeded the recommended limit.

Nevertheless, says Mr. Veillette, “These are very well ventilated environments. The hourly air change rate meets the standards, but because the sewage is more contaminated, the ventilation can’t keep up with the increase in biomass."

Virus Bellwether

Workers may also be exposed to pathogenic viruses, including influenza during flu season. However, the researchers were looking for a virus that is always present, as this can be a good indicator of total viral exposure in a workplace.

Through molecular techniques, a virus exposure biomarker was selected: the adenovirus. "This virus acts as a bellwether, signaling the presence of viruses in the air. If a plant implements strategies to improve air quality, we can know right away if they’re effective by measuring only the presence of adenovirus, rather than all viruses," explains Ms. Duchaine.

Sick More Often?

The researchers took advantage of being in the treatment plants to verify a piece of information frequently reported: that the people who work there complain of being sick more often than the general population. About 20 of these workers were monitored for a year and their results were compared with those of a control group.

They were found to have more frequent gastrointestinal symptoms, particularly nausea, intestinal upset, abdominal pain and heartburn. In terms of respiratory symptoms, only earaches and expectoration were more common in the plant workers than in the control group.

These initial results put a bug in the ear of the research team, who plan to continue their epidemiological monitoring in order to provide a more complete picture of the health of these workers and the risks associated with working in a sewage treatment plant.