Garbage bins containing separated organic leftovers such as banana skins, potato peelings and apple cores release far higher numbers of potentially harmful bacteria and molds into the kitchen than those containing mixed garbage, Dutch researchers have found.

Netherlands researchers say that the contaminants include bacteria and molds known to aggravate common respiratory ailments, such as asthma. Millions could be at risk when they dutifully segregate organic materials from their household waste for collection and recycling as compost.

Researchers made the discovery after collecting and analyzing household dust from 99 Dutch households, some with segregated and some with mixed waste. They decided to carry out the study after complaints from people when segregation was introduced in the Netherlands six years ago. There were loads of complaints about the smell from organic bins, and some people with respiratory illnesses said their symptoms got worse. The reason is that the humid state of pure organic waste suits microorganisms better than mixed waste, which "dilutes" the organic material.

The team tested for cell fragments called endotoxins, which are released when bacteria die. Their levels reflect concentrations of bacteria in the dust. If exposure to these is high enough, the endotoxins themselves cause coughing, breathlessness and some flu-like symptoms. To detect molds, researchers screened dust for beta-glucans -- sugars found in the coatings of numerous fungi. Beta-glucans have been shown to cause lung and throat inflammation. The team also screened for exopolysaccharides, polymers found in the cell wall of common Aspergillus and Penicillium molds.

In homes where organic bins regularly remained unemptied for a week or more, levels of the bacterial endotoxins were three times as high as in homes with unseparated waste. Levels of the mould materials, exopolysaccharides and glucans, were even higher, some up to eight and five times as high respectively compared with households that mix their waste. In houses with dust-accumulating textile floors, as well as an organic waste bin, concentrations of the microbial agents were higher still -- anything from 25 to 840 times as high as in houses with neither risk factor.

The solution is to keep the organic waste bin outside, but in many cases that's not possible in downtown areas, where there's less space. The other remedy is to empty the bin more regularly into the larger, roadside receptacles provided by the authorities.

Applied and Environmental Technology (vol 66, p 627) March 2000