The Conservation Law Foundation yesterday sued the Encore Boston Casino and four bus companies over casino shuttle buses it charges were allowed to idle for more than the state-allowed five minutes at locations in Everett and Medford, including the Wellington T stop.

In its suit, filed yesterday in US District Court in Boston, the foundation said it sent observers to the T stop and Mystic Street in Everett, near the casino, last fall and found 301 instances of buses idling for longer than five minutes - in one case at Wellington for two hours and 36 minutes.

The five-minute rule is part of a state regulation adopted as part of an agreement with the EPA under the federal Clean Air Act. The foundation charges that letting the casino's diesel buses idle for longer than allowed releases "fine particulates, nitrogen oxides, sulfur dioxide, benzene, formaldehyde, and forty other kinds of toxic air contaminants" that can harm nearby residents, elementary-school students, parks and wildlife.

Fine particulates have been found to impair lung function, aggravate respiratory illnesses such as asthma, bronchitis, and emphysema, and are associated with premature deaths. Exposure to NOx can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms (such as coughing, wheezing or difficulty breathing). Longer exposures to elevated concentrations of NOx may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. People with asthma, as well as children and the elderly, are generally at greater risk for the health effects of NOx. Exposure to SO2 can cause respiratory illness, including aggravation of asthma, other adverse effects on breathing, and alterations in pulmonary defenses, as well as aggravation of existing cardiovascular disease.

In its complaint, the foundation asks a judge to order Encore's owner and the bus companies to stop letting buses just sit with their engines running and pay $99,681 for each day where buses were found to be idling for more than five minutes.