At least 77 people reported severe respiratory problems over a one-week period at the end of March — including six who were hospitalized with pulmonary edema, or fluid in the lungs — after using a “Magic Nano” bathroom cleansing product, according to the Federal Institute for Risk Assessment in Berlin.

Symptoms generally cleared up within 18 hours, though some had persistent breathing problems for days.

Studies of health effects have just begun in several countries, and regulatory agencies are still formulating their stances, but hundreds of nano products are already for sale.

It was unclear yesterday what kind of nanomaterial is in the spray, or even whether the particles were to blame. Every case has involved the aerosol spray-can form (the product was previously available in a pump bottle, without complications). And the propellant used in the aerosol has long been used uneventfully in hair sprays and other products.