Earlier this summer, Destiny Valencia and her fellow baristas at a Starbucks in San Jose, Calif., were learning to make a new line of specialty drinks when they began to feel sick.

The drinks, called blended refreshers, were billed as a “smooth, frozen twist” on the neon-hued, fruit-speckled iced drinks that young people posted on TikTok.

The new refreshers included an ingredient called “blending powder,” meant to preserve the drink’s slushy integrity. But as soon as Ms. Valencia started handling the powder, there was a problem: It went airborne.

“It kind of creates a smoke cloud,” Ms. Valencia said. Among her colleagues, she added, “there were thoughts of — is this OK to breathe in?”

Soon, orders were pouring in for blended mango dragonfruit lemonade refreshers and blended blue coconut lemonade refreshers. Ms. Valencia started getting headaches.

“Any time I had to use it, I had to hold my breath,” she said. Her colleagues complained about coughs and tickling throats. Baristas around the country reported developing hives and having asthma attacks.

This week, Starbucks removed the blending powder from its U.S. stores.

Jaci Anderson, a spokeswoman for the company, said blended refreshers had been removed in a regular seasonal menu transition. She said Starbucks had no plans to bring back the powder.

“Partners and customers can feel confident in the ingredients we use,” Ms. Anderson said.

Workers United, the union representing thousands of Starbucks employees at more than 700 of the nearly 17,000 Starbucks stores in the United States, called the move a victory, won through worker complaints and outraged social media posts.

“We know our collective action prompted the company to direct stores to remove the powder by the end of August,” said Jasmine Leli, a union barista and bargaining delegate from Buffalo. She said the union wanted Starbucks to consult baristas before introducing ingredients that could affect their health.

The blended refreshers are a bright, frothy reminder of the challenges Starbucks faces: tight competition for consumer attention, a protracted labor dispute and consumers more attuned to the potential health risks of food additives.

Refreshers are a fast-growing business for restaurant chains. The brightly colored, sweet drinks are showing up on menus everywhere, in a race to create unique, or bizarre, versions of the drinks to capture attention on social media and in consumers’ imaginations.

For Starbucks, refreshers are a $2-billion-a-year business and an important part of a turnaround under Brian Niccol, the chief executive, who joined the company nearly two years ago.

“Refreshers remained a standout platform for us,” Mr. Niccol said on an earnings call in July. He said the drinks had delivered double-digit annual revenue growth and encouraged customers to visit the chain throughout the day.

Refreshers and Frappuccinos, the refreshers’ increasingly elaborate elder cousins, have become the bane of existence for many baristas. Customers often request customizations that make the drinks laborious to prepare. Unless baristas add a binding agent, such as the cream used in Frappuccinos, blended drinks can separate within minutes into an unappetizing diptych of discolored ice and luminous juice.

Enter the blending powder.

It “was flying off the shelves,” said Nick Parker-Smith, a shift supervisor at a Starbucks in Springfield, Mo. He meant this figuratively and literally: The powder, he said, “becomes airborne quite easily.”

“We really weren’t equipped to handle this,” Mr. Parker-Smith added.

Baristas complained about the powder on Reddit and in social media videos, describing rashes and breathing problems, sharing ways to protect themselves from the cloud that kicked up when they used the powder, especially when it was poured into countertop containers from the plastic bags in which it arrived.

A complaint about the powder’s use at a Starbucks in Maryland was filed with the Occupational Safety and Health Administration, according to the union and public records. The company said the complaint had been resolved.

Barista concerns about the powder grew when social media posts drew attention to the ingredients, which included a small amount of amorphous silica, an additive that prevents clumping and that the Food and Drug Administration has approved for consumption.

Crystalline silica, a different form of the same chemical compound, can cause a lethal lung disease, silicosis, when inhaled as a fine dust. It has been linked to a health crisis among countertop manufacturers working with artificial stone, which often contains large amounts of the material.

Amorphous silica has not been linked to the same illness, and has been proposed as an alternative to crystalline silica in countertops. But doctors generally advise against breathing any kind of dust for extended periods, at a minimum because of irritation in the airways.

Two weeks after the blended refreshers were introduced in July, Starbucks posted a note on its website saying the blending powder was safe. The company also released guidance to baristas on how to transfer the powder to the countertop containers, urging them to pour slowly.

“It’s safe for use in food and safe for use in our coffeehouses when used according to instructions,” Ms. Anderson, the Starbucks spokeswoman, said.

Workers United asked Dr. Peter Orris, an occupational health physician at the University of Illinois, for feedback. In a letter to the union reviewed by The New York Times, Dr. Orris said that the powder contained ingredients that might be deemed safe, but that Starbucks “should have evaluated it with respect to its impact on human health prior to introducing it in a setting exposing both employees and customers to the aerosolized powder through both inhalation and skin deposition.”

Dr. Orris said there appeared to be no lasting consequences to human health, aside from irritation during exposure, though the effects were worse for people with asthma or allergies.

The union sent the letter to Starbucks headquarters in Seattle.