Emergency services were called to Silverburn shopping centre in Glasgow just after 12:30pm today after reports some people were feeling unwell.
Police said about 50 people were receiving medical treatment as a precaution.
The Scottish Fire and Rescue Service (SFRS) sent a specialist environmental protection unit to the scene along with a number of fire engines.
Linsey Hanna was at the centre when it was beginning to shut down.
The 22-year-old from Glasgow said: “Alarms went off for the entire shopping centre.
“There were quite a lot of workers getting treated outside by paramedics. Police then started asking if we had any coughing problems.
“We heard one worker say they had known something wasn’t right.
“There were hundreds of people, it was actually crazy. Every single person was evacuated.
“Outside, it was just chaotic. It was absolute pandemonium.”
In a statement posted on social media, Silverburn said: “We’ve temporarily evacuated Silverburn as a precautionary measure and the emergency services are in attendance.
“The safety of our customers and staff is always our number one priority. We will keep you updated on the situation.”
Shoppers also took to Twitter to describe their experience.
One person called Caroline tweeted: “In silverburn over an hour ago myself and lots of others went into fits of spontaneous coughing, reported this to a silverburn staff member.”
Another person wrote: “Silverburn has been evacuated, people randomly coughing.”
A number of road closures were also put in place for at least an hour-and-a-half due to the incident, including Barrhead Road, Peat Road and the M77 J2 off-slip.
Glasgow City Council announced the restrictions had been lifted about 4:25pm.
An SFRS spokesman said: “The Scottish Fire and Rescue Service was alerted at 12:34pm on Wednesday to reports of a chemical smell at Silverburn shopping centre, near Glasgow.
“Operations control mobilised a number of fire engines and a specialist environmental protection unit.
“Crews ensured the area was safe and have now left the scene.”
A Police Scotland spokesman said the centre reopened about 4pm, but some shops have said they will not be back to business for the rest of the day.