Nearly 2,000 pilots at four cargo carriers—Atlas Air, Southern Air, Polar Air, and ABX—are voting this week to authorize the union to call a strike.

Pilots at a fifth carrier—Kalitta Air—have already voted overwhelmingly authorizing the union to a strike.

All five carriers are part of DHL’s North American network.

The pilots are represented by the International Brotherhood of Teamsters and its airline affiliate, Teamsters Local 1224, and have been stalled in contract negotiations with airlines, in some cases for years. The four new strike votes will be completed within the next 10 days. The strike authorization at Kalitta was approved by 97 percent in December 2015.

The five airlines play a major role in linking DHL’s highly profitable North American express service to the rest of the world, and make up an estimated 70 percent of DHL’s total annual freight ton miles.

According to a study conducted by Teamsters Local 1224, DHL contracted pilots are paid considerably less and work much longer hours than pilots who fly for U.S. logistics companies UPS and FedEx. Pilots at Atlas, Polar and Southern report being forced to fly long hours with minimal rest time in between flights, leading to dangerous fatigue, according to the union.

“No one wants to go on strike, but we can’t continue operating under our current workplace rules which keep pilots on duty for almost 28 hours without rest,” said Captain Bryan Holmberg, a Southern Air pilot.  “It’s time the company finally stop putting the safety of our pilots and the public at risk just to increase profits.”

According to the Teamsters, all of the carriers have seen profits rise in recent years. Air Transport Services Group–which owns ABX–recently announced an agreement with Amazon.com to operate an air cargo network for customers in the U.S.

ABX pilots endured furloughs and wage and benefit concessions in 2009, when DHL abruptly cut its operations in Wilmington, Ohio. Since then, pilots have been working under the 2009 concessionary contract with ABX and have been negotiating for an amended contract for more than two years. The parties’ negotiations are deadlocked, according to the union, over the company’s refusal to provide the ABX pilots with industry-standard job security protections and its disruptive practice of transferring work between ABX pilots and pilots at a sister company.

Three of Atlas Air Worldwide Holdings’s four air carrier subsidiaries (Atlas Air Cargo, Polar Air Cargo and Southern Air) and their pilots are in the middle of bargaining for new collective bargaining agreements. Earlier this year, the negotiations took a turn when AAWW announced its acquisition of SAI, and its two air carriers, Southern Air and Florida West.

According to the Teamsters, AAWW and Atlas Air insisted after completing the SAI acquisition, that Atlas Air and Polar pilots stop ongoing contract negotiations and merge their contract with the Southern Air pilots’ existing contract. The Teamsters, in a statement said that “move that would have a devastating impact by suppressing wages and lowering quality of life issues for pilots at the three companies and throughout the industry.”

AAWW says that existing collective bargaining agreements calls for the merger of those agreements in the event of an acquisition.

The Atlas and Polar pilots filed for mediation with the National Mediation Board.

Pilots at Kalitta Air, Inc., have already voted overwhelmingly to authorize the union to call a strike against their employer.  The Teamsters, representing the Kalitta Air pilots, have been engaged in negotiations with the company for nearly six years.