With averages greater than 30ppb, lagoons have high concentration of hydrogen sulfide, which is highly toxic. A study by the Minnesota Pollution Control Agency has found that concentrations of Hydrogen sulfide near lagoons have exceeded the state standard, even as far away as 4.9 miles. Hydrogen sulfide is recognizable for its unpleasant rotten-egg odor. Because hydrogen sulfide is heavier than air, it tends to linger around lagoons even after ventilation. Levels of hydrogen sulfide are at their highest after agitation and during manure removal.