Levels of 50 or 100 should be regarded as a serious problem. And 200 ppm or above indicates that the TLV-C has been exceeded. If the unit alarms, emergency services should be called. Curtis referenced studies showing that traditional open burning of leaves in the fall generates dangerously large quantities of carbon monoxide, particulates, and at least seven proven carcinogens. Several medical journal papers also showed that leaf-burning pollution severely increases breathing problems in a majority of asthmatics.
Toxic smoke is emitted by all natural organic substances.
It matters little if you burn coal, oil, wood, incense, hamburger, or tobacco. Just because we like the smell of incense or burning autumn leaves does not make their smoke healthier.