QUESTION: Hey Dr Zac, over winter I was cranking up the heater, but someone at work mentioned these reverse cycle heaters/air conditioners, especially older ones, can spread bacteria like legionella. We also have a gas heater in the kitchen, and he said that can emit carbon monoxide. Is that true? With summer coming up - should I stop using our reverse cycle airconditioning. What should I use instead!? Melbourne can get putrid during the hot months. What's the safest way to stay warm and cool without risking my health? - Carla, 42, Dandenong VIC

ANSWER: Dear Carla, thanks for your question. Melbourne's four-seasons-in-a-day climate means our heaters and airconditioners work overtime but do they put our health at risk? Let's clear it up.

Air-conditioners and legionella

Legionella is the bacteria behind Legionnaires' disease, a severe pneumonia. It thrives in stagnant warm water that gets dispersed as a mist, which is why outbreaks are linked to cooling towers, spas and fountains. Your reverse-cycle split system doesn't store water, so it isn't a breeding ground. The main risk at home is dust, mold and allergens in dirty filters, which can flare up asthma or hay fever. Regular cleaning fixes this.

Gas heaters and indoor air quality

Gas is a different story. Unflued or poorly maintained heaters can release carbon monoxide, which is a colourless, odorless gas that at high levels can be deadly. Even at lower levels, long-term exposure is harmful. Gas appliances also produce nitrogen dioxide, which research has repeatedly linked to asthma flares in children and lung irritation in adults. Health authorities and peer-reviewed studies both point to this as a measurable risk.

The good news: a properly flued and serviced heater, in a ventilated room, is much safer. But if yours is old, unflued, or hasn't been checked in years, get it inspected. And a carbon monoxide alarm is a smart investment.

How to stay safe

* Stick with reverse-cycle airconditioning. It's efficient, safe and low-pollution when serviced.

* Insulate and draught-proof to reduce how hard your systems need to work.

Carla, this means you can keep using your reverse-cycle without worrying about legionella. Focus on your gas heater because that's where the real evidence points to risk.

Is this household device linked to Legionnaires' disease?