Around dinner time yesterday, sirens wailed in Sarnia, Ontario, warning residents to get inside as a cancer-causing chemical leaked into the air from a nearby Shell refinery.

People shut their doors and windows tight to stop the benzene, a sweet-smelling, highly-flammable carcinogen, from seeping into their homes.

Roads near the plant were temporarily closed, but have since re-opened. Residents are now free to leave their homes. Air monitoring is underway and crews are investigating the cause of the leak, according to a hazardous materials alert.

It's an all-too-familiar song and dance for locals, who have dubbed the industry-saturated area "Chemical Valley." The region is home to about 40 percent of Canada's petrochemical industry, according to a 2007 Ecojustice report, and in 2011 the World Health Organization crowned Sarnia's air the worst in Canada. The air smells of gasoline, asphalt, and bad eggs, according to VICE Canada's documentary on the issue.

"It does happen a lot, but I think yesterday's was a little bit scarier because it was benzene and they sounded the alarms for a long time and people were freaking out a bit more than a usual spill," Vanessa Gray told VICE News of the frequent industrial hazard alerts her community experiences.

For years, the resident of Aamjiwnaang First Nation has been raising awareness about the toxic industry next door to the reserve.

'It does happen a lot.'

On her way to dinner at her family home Wednesday, Gray noticed a heavy smell in the air. "But that's normal," she said. "Once in awhile you always smell something weird."

When the alarms went off during dinner, they didn't stop, which meant it wasn't a drill. Gray's family made sure the windows and doors were closed, tuned into the radio for updates and played Yahtzee until the "shelter in place" warning was over.

"It was really scary because I was there with my family, and I saw their faces and could tell they were a little afraid," she said.

Found in crude oil, gasoline and cigarette smoke, benzene can cause anemia by preventing bone marrow from producing enough red blood cells, and can also damage the immune system, according to the Centre for Disease Control and Prevention (CDCP).

The World Health Organization states that benzene causes cancer, and "no safe level of exposure can be recommended."

Inhaling high levels of benzene can cause drowsiness, dizziness, irregular heartbeat, headaches, tremors, confusion and unconsciousness, the CDCP says, and in very high levels, benzene can kill.

Other than causing cancer, long term exposure to benzene can cause excessive bleeding and can increase the chance of infection. For women, inhaling benzene can lead to irregular periods and shrink their ovaries.

Aamjiwnaang residents say they have higher rates of cancer, and Gray says her family members have been directly affected.

According to the Globe and Mail, hospital case data from the 1990s showed the overall cancer rate for men living in Sarnia was 34 percent higher than the provincial average.

Shell spokesperson Cameron Yost told VICE News the company is continuing its investigation into Wednesday's incident, and the "shelter in place" warning was "a precautionary measure." Air quality in Sarnia is now normal, Yost said.

"The safety of our community members and employees remains our number one priority, and we sincerely apologize for any inconvenience our neighbors may have experienced during this incident," Yost said.

It's not the first time a leak at the Shell plant near Sarnia has set off alarm bells.

In January 2013, the same plant leaked gas that caused residents to complain of irritated eyes, sore throats and nausea, according to the Sarnia Observer.

'We sincerely apologize for any inconvenience our neighbors may have experienced during this incident.'

The leak of mercaptan, which smells of dirty socks but isn't dangerous, was caused by corrosion in a line at the Shell refinery. The company was also charged with releasing hydrogen sulphide, the Observer reported, but the charge was dropped.

The company pleaded guilty and was charged $500,000. The company also had to pay $200,000 to Aamjiwnaang.

The company's lawyer said in court the company regretted the incident, had learned from the leak and had put preventative measures in place to stop it from happening again, the Observer reported.

But Shell isn't alone. In Sarnia last year, there were 16 alerts of hazardous material leaks, industrial fires and air quality warnings. Spilled and leaked substances included hydrogen sulfide, methyl chloride, hydrocarbons and benzene.

If industry causes harm to the people nearby or the land, they should shut down operations altogether, Gray argues.

"Indigenous people have a right to use their land, and if they can't and there are sirens going off all the time, that means a treaty has been broken," she said.

Follow Hilary Beaumont on Twitter: @HilaryBeaumont