Air pollution toxicology and public health: Carbon monoxide (CO) in ambient air can originate from natural decomposition of chlorophyll and from haemoglobin breakdown in some animals, in addition to large anthropogenic sources such as incomplete combustion. When CO concentrations exceed the threshold limit value (TLV), commonly taken as > 50 ppm for short-term workplace exposure, what is the principal health impact on humans?

Introduction / Context:
Carbon monoxide (CO) is a colorless, odorless gas produced by incomplete combustion of carbonaceous fuels and also by certain natural processes. Because CO binds strongly to haemoglobin in the blood, exposure above safe limits can rapidly impair oxygen transport. This question tests recognition of the dominant human health effect when concentrations exceed a typical threshold limit value (TLV) on the order of 50 ppm for workplace contexts or much lower for ambient guidelines.

Given Data / Assumptions:

• CO concentration considered is beyond TLV (greater than about 50 ppm).
• Exposure is acute or short term, sufficient to cause clinical effects.
• Population is assumed to be unprotected and breathing ambient air.

Concept / Approach:
CO has a high affinity for haemoglobin, forming carboxyhaemoglobin (COHb). This competitively inhibits oxygen binding and shifts the oxygen–haemoglobin dissociation curve, reducing tissue oxygen delivery. The net physiological outcome is hypoxia, presenting as headache, dizziness, confusion, loss of coordination, and at higher levels, loss of consciousness and death by asphyxia. CO is not a meaningful greenhouse gas on policy timescales, nor does it directly alter sea level.

Step-by-Step Solution:
Identify the toxicological mechanism: COHb formation reduces blood oxygen-carrying capacity.Relate mechanism to symptomatology: hypoxia leads to asphyxial outcomes.Select the option that states the principal health effect: asphyxia.

Verification / Alternative check:
Clinical guidance links percent COHb to symptom severity; values above 10–20% produce neurological symptoms, and higher levels can be fatal. This aligns with the characterization of CO as an asphyxiant rather than as a greenhouse gas of concern.

Why Other Options Are Wrong:
Greenhouse/temperature increase and sea level rise: CO is not a major radiative forcing gas; these outcomes are associated with CO2 and other greenhouse gases.Enhances greenhouse effect: While CO can indirectly affect atmospheric chemistry, the immediate health concern at TLV exceedance is asphyxia.

Common Pitfalls:

• Confusing CO with CO2 in climate discussions.
• Underestimating danger due to CO being odorless and colorless.

Final Answer:
causes asphyxia