Here is Morantz's conclusion and suggestion for further study:
I find that unionization is associated with a sizable and robust decline in both traumatic
injuries and fatalities, the two safety outcomes that I argue are the least prone to reporting bias. I
construe these results as evidence for a “real” union safety effect in U.S. underground coal
mining. At the same time, I find that unionization is associated with higher total and nontraumatic
injuries, lending credence to claims that injury reporting practices differ significantly
across union and nonunion mines.
Interestingly, the union safety effect on traumatic injuries seems to have escalated just
before the turn of the millenium. I propose several possible explanations for this trend, including
an overall improvement in labor relations since the 1970s, fluctuations over time in the
stringency of MSHA’s enforcement scrutiny, the growing competitive pressures faced by union
leaders, and the increasing sophistication and professionalization of UMWA safety programs.
The empirical evidence available, although scant, suggests that the latter hypothesis is the most
promising. Exploring the historical relationship between UMWA activities and mine safety in
greater detail – including a richer, updated institutional account of the precise mechanisms
whereby organized labor affects safety outcomes – would be a promising topic for future inquiry.
While the article is about mine safety, I am open to discussing the role of unions in promoting safety in all workplaces.