About 16.7 million of the faulty Takata air-bag inflators still haven't been replaced about three years after federal regulators began to oversee a broad-ranging recall of the defective inflators that have caused at least 15 deaths in the U.S., according to a new report.

The Takata airbag recall — one of the largest in history — involves 19 automakers and as many as 50 million airbag inflators scheduled to be recalled by December. The number of total inflators that need to be repaired will grow in January, when 10 million more defective inflators are added to the recall, according to a Dec. 21 update from independent monitor John Buretta.

Year to date through October, automakers repaired more than 7.2 million defective inflators, which represents about 28% of the inflators in the recall repaired to date. Many automakers have now repaired between 40% to 70% of the defective inflators in their vehicles, according to Buretta.

"The past year has been marked by significant, industry-wide advancements in tackling the Takata recalls as more vehicle manufacturers have adopted key strategies for effective outreach," Buretta wrote.

The Takata devices can explode with too much force and hurl shrapnel at drivers and passengers. They use the chemical ammonium nitrate to create a small explosion that inflates the air bags.