Justin is right, you can't avoid pollution and kelp has a much better ratio of the required electrolytes.
You will not get much benefit from using spirulina and you may subject yourself to a loss of B12!
The kelp I buy from seaveg.com is tested for toxins, they sell clean and safe sea weeds!
We have had nothing but positive results from using it, but as with anything it should be used moderately because you consume it everyday.
Start slow, don't overdo the ingredients, start with trace amounts, then find your tolerance level and stick with it.