I noticed that API-14C recommends monitoring of 2-ppm (and higher) SO2 levels. I felt this was a really low threshold value for SO2 .
What surprised me further is that the Immediately Dangerous to Life or Health (IDLH) value for SO2 is 100-ppm…same as H2S!
Intuitively, I did not think SO2 was as toxic as H2S (for comparable exposure time). Definitely, not for acute exposure.
Extended exposure to SO2 can lead to pulmonary edema which can be fatal. So I can understand the ERPG-3 (60-minute exposure) value for SO2 (15-ppm) being lower than that for H2S (100-ppm).
Let us see what the LC50 values are are telling us – LC50 is the concentration of a chemical or substance which would produce death in 50% of the test subjects by inhalation route of exposure.
Here are LC50 values adjusted to 0.5-hour exposure:
- H2S : 350 ppm
- SO2 : 670 ppm
LC50 for SO2 is almost two times higher than H2S i.e. H2S is more potent than SO2.
So why is H2S IDLH same as SO2? Why SO2 monitoring levels are as low as 2-ppm?
The only explanation I can think of is that the toxic threshold values are recommended by an expert committee. Knowing SO2 can have an irritating effect within the respiratory tract at lower concentrations, they must have recommended a much lower value that LC50.