Cyanobacteria, and their toxins, are incredibly diverse. How can risk be measured? 

When a greenish cloud, surface scum or other suspicious algae appears in a previously clear lake, the message is clear: Stay away. As cyanobacteria blooms become more common in New Hampshire, that precaution has become common practice across the state.

The reason? Cyanobacteria, an ancient single-celled organism that multiplies to excess when nutrients and heat are plentiful, can produce toxins harmful to the skin, liver, nervous system, stomach and more. Effects range from mild illness and rash to, in rare instances, death; therefore, authorities like the New Hampshire Department of Environmental Services and the Centers for Disease Control and Prevention urge would-be swimmers, paddlers and pets to stay away from water that shows signs of a bloom.