Researchers from the California Department of Health Services investigated whether there was any link between rice burning and daily asthma hospitalizations in Butte County, California between 1983 and 1992. (Since the 1920s, farmers have used fire to rid their fields of waste rice straw that was difficult to plow under in wet heavy fields.)
They reported that on days when more than 500 acres were burned, the rate of hospitalization for asthma increased by 23 percent (relative risk 1.23, 95 percent confidence interval 1.09 to 1.39). On days when between 100 acres and 499 acres were burned, asthma hospitralizations increased by 20 percent (relative risk 1.2, 95 percent confidence interval 1.05 to 1.37). No statistically significant increase was reported for less than 100 acres burned.
Based on these results, the researchers concluded:
This evidence suggests that further limitations on the daily amoujnt of rice straw permitted to be burned should be considered to reduce pulmonary morbidity related to asthma.
But there are two key problems with their conclusion.
First, airborne particulate matter was poorly correlated with daily totals of rice stubble burned (i.e., a 0.04 Spearman rank correlation where 0 is no correlation and 1 would be the strongest correlation).
Second, there was no statistically significant association between airborne particulate matter and asthma hospitalizations!
So there's no biological link between rice burning and asthma hospitalization.
This study is yet another example of how a statistical association doesn't necessarily equate to a cause-and-effect relationship.
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