Hot yoga classes are soaring in popularity. A number of celebrities, including Lady Gaga and Madonna, swear by hot yoga, along with legions of yoga practitioners. But is exercising in extreme heat and humidity healthful? Consumer Reports medical experts took a closer look at the trendy exercise and are cautioning those who take part in the classes.

Many of the hot yoga classes require heat of at least 40°C and humidity around 40 per cent. Though there is little specific research on hot yoga, exercising in extreme heat can cause a number of uncomfortable and even dangerous symptoms. It can lead to heat exhaustion or heat stroke. Warning signs include feeling lightheaded or dizzy, or experiencing nausea or muscle cramps. If you suffer more serious symptoms either during or after class, such as unusual weakness, fever, vomiting, or confusion, Consumer Reports advises going to the nearest emergency room.

Bikram Yoga practitioners say the high temperature and humidity promote health, and studio owner Rich Pike says he hasn’t had complaints of heat exhaustion.  Instead, he touts the benefits.

“Heat allows you to bend safely and be more flexible. What the sweating does is it eliminates toxins through your sweat,” he said.

Consumer Reports says that though the heat may help you stretch further, it can also cause you to overstretch, leading to possible joint or muscle damage. The best advice: Whatever exercise you do, stop if you feel pain or heat exhaustion and be sure to always drink plenty of water.

With any type of yoga, Consumer Reports says there are steps you can take to avoid picking up viruses or bacteria. Bring your own mat and towels. Cover any cuts or scrapes with an adhesive bandage, and use alcohol wipes to wipe down any surfaces, such as mats or blocks.