Trekkers will be exposed to poisonous vapour and volcanic ash should there be any volcanic activity. Hence, it is best to make your arrangements through a reliable travel operator. Be sure to keep in constant contact with them prior to your travel/trekking dates, to make sure it is a “safe time” for your trek.
This not only means your guide’s instructions. If there are signboards posted up in the trekking area prohibiting trekkers from doing certain things, or going beyond certain areas, there is usually a good reason, such as preventing injuries or even fatalities.