On 2014 Nov 25, David Mage commented:

The authors mention "Mortality rate ratios for PM2.5 fluctuated over time, but without clear trends despite a substantial drop in the sulfate fraction," which led to the prior published comment "Is ambient PM2.5 sulfate harmful?" Perhaps they all have forgotten the historic work of Mary Amdur (PMID 679907, 2667973) who wrote "The irritant potency of the sulfate species varies so widely that the term 'suspended sulfate' is toxicologically meaningless," and "Sulfate is an unsatisfactory surrogate in existing epidemiological studies."

On 2014 Nov 25, David Mage commented:

The authors mention "Mortality rate ratios for PM2.5 fluctuated over time, but without clear trends despite a substantial drop in the sulfate fraction," which led to the prior published comment "Is ambient PM2.5 sulfate harmful?" Perhaps they all have forgotten the historic work of Mary Amdur (PMID 679907, 2667973) who wrote "The irritant potency of the sulfate species varies so widely that the term 'suspended sulfate' is toxicologically meaningless," and "Sulfate is an unsatisfactory surrogate in existing epidemiological studies."