Duh, it is pollution and it is toxic.

What Happens When Wood Burns?

Complete combustion gives off light, heat, and the gases carbon dioxide and water
vapor. Because when wood burns complete combustion does not occur, it also
produces wood smoke, which contains the following major air pollutants, which are
regulated by State and Federal regulations because of their known health effects:
Carbon Monoxide (CO) – An odorless, colorless gas,
produced in large amounts by burning wood with insufficient
air. CO reduces the blood’s ability to supply oxygen to body
tissues, can cause stress on your heart and reduce your ability
to exercise. Exposure to CO can cause long-term
health problems, dizziness, severe headaches,
unconsciousness and other serious effects. Those most at risk
from CO poisoning are the unborn child, and people with
anemia, heart, circulatory or lung disease.

Oxides of Nitrogen (NOx) – NOx impairs the respiratory system and its ability to
fight infection. NOx also combines with VOCs and contributes to the formation of
ozone and with water vapor to form acid rain or acid fog.

Volatile Organic Compounds (VOCs) – Evaporated carbon compounds which react
with NOx in sunlight to form ozone (photochemical smog). Ozone injures the lungs
and makes breathing difficult, especially in children and exercising adults. NOx and
VOCs also form particulate matter through a series of complex reactions.
Toxic Pollutants – Wood smoke also contains VOCs which
include toxic and/or cancer-causing substances,

such as
benzene, formaldehyde and benzo-a-pyrene, a polycyclic
aromatic hydrocarbon (PAH). Manufactured fireplace logs, for
instance, are not recommended for burning because they
produce toxic fumes, including PCBs (polychlorinated biphenyls). Researchers are
now studying these and other smoke products to learn more about their effects on
human health.
Particulate Matter less than 10 microns in
diameter (PM10) are very small droplets of
condensed organic vapors of wood tar and
gases. These particles are a result of unburned
fuel and have a diameter of 10 microns or
smaller (the diameter of a human hair is about
50 to 100 microns), which allows them to be
inhaled into the lungs. Exposure to PM10
aggravates a number of respiratory illnesses.
PM10 includes a smaller group of particles called PM2.5, particles with diameters of
2.5 microns and less. These finer particles pose an increased health risk because
they can lodge deep in the lungs and contain substances that are particularly harmful
to human health, contributing to lung diseases and cancer. Exposure to PM2.5 may
even cause premature death in people with existing heart and lung disease.