The gases emenating from Hawaii's volcanoes are cool -- so unique they even have their own name: vog (for volcanic smog). But they also can be a health hazard -- especially on breezeless days. In fact, the residents of the nearby town of Volcano actually have the same respiratory problems you would expect of someone living in Seoul or Beijing, including a higher than average number of child asthma cases. As a healthy visitor, you shouldn't have any problems, but if you have chronic lung issues, it's best to avoid the park on voggy days. The visitor center will post warnings on those days, as you can see in the adjacent photos.
When you go to the volcanoes, you can walk to near where the lava goes into the ocean. You start walk there late afternoon and then wait until it's dark so you can see the glowing lava. There are ropes everywhere and you need to stay behind them. People have gone missing here before because "new lava" is very unstable and sometimes whole parts just drop into the ocean. Also, when you return it will be pitch black and you need a (good) torch to get back and it might take up to 2 hours. I ran into someone who had fallen and was pretty cut up from the lava. Not trying to scare you off, but if you feel unsure about this, I'd recommend a helicopter ride instead...
Signs around calderas and volcano warn you not to expose yourself for too long in these areas. Sulphorous fumes are emitted on a constant basis and breathing them can be extremely hazardous to your health if exposure is prolonged.
The Volcano Park is situated on two active volcanoes. You need to be aware of possible hidden dangers associated with volcanic environments. Wear adequate clothing and sturdy shoes; stay on designated trails and don’t enter closed areas or lava tubes. Carry some water with you and sunscreen. Volcanic fumes are hazardous and can also be life-threatening.
It is recommended that if you have heart or breathing problems, pregnant or you have infants and young children then you are especially at risk and should avoid stopping at Sulphur Banks, Halema'uma'u Crater and other areas where fumes are present.