In May of last year, Mayor De Blasio announced plans to build 80,000 new units of affordable housing over the next decade. Yet, as the mayor moves ahead with this ambitious housing agenda, he is facing a dilemma: the construction unions are pushing to ensure that it is their members who build most of these new units, or, at at minimum, the workers are paid higher wages. At the same time, affordable housing developers say they won’t be able to meet the mayor’s housing goal if they are forced to hire union laborers.

“It is really a question of the math,” said Jolie Milstein, the president and CEO of the New York Association for Affordable Housing. “If you look at the cost of constructing affordable housing, particularly in an environment of increasing land costs, you just get very many fewer units built if you have to use expensive union labor.”

For their part, the unions point to reports that indicate the majority of construction accidents occur at non-union sites, and they say non-union companies fail to pay their employees fair wages and do poorer quality work.

“What these non-union contractors are doing is, they’re exploiting their workers in a way, that is almost like their indentured servants,” said Gary LaBarbera, the head of the Building and Construction Trades Council, which represents 100,000 union construction workers. “They use them until their usefulness is no longer and then they get rid of them and replace them.”

It is difficult to verify Mr. LaBarbara’s allegations because unlike the unions, non-union firms don’t publicize their wages. The construction industry is known for its lack of transparency, leading to the conflicting studies, and information can often be unreliable. Safety records, for example, are often dependent on companies self-reporting their accidents because there are not enough inspectors to visit every building site. And while the unions cite reports showing non-union construction sites are less safe, Milstein touts a report her organization recently published that found the opposite.

Even the agencies responsible for tracking safety don’t always agree. The federal Occupational Safety and Health Administration, or OSHA, reported 13 construction-related deaths through August of this year, 11 of which were at non-union jobs. But New York City said there have only been 7 fatalities, and it doesn’t track union affiliations.

For a mayor who wants more affordable housing, as well as jobs that pay better wages, it could be a difficult balancing act.

Correction: An earlier version of this story misspelled Gary LaBarbera's last name.