The Color of Smog

By Lucy Randel, , M.S., B.S.E., Air Alliance Houston Board Member

Air pollution is a complex phenomenon that confuses many because sometimes you see it and sometimes you don’t. And sometimes you think you see it, but can’t be sure. Even our own staff sometimes get confused with the terminology.*  So, if we have a high ozone day, will the air look different? The answer is yes and no. No, because ozone itself is a colorless gas. Yes, because high ground level ozone levels are an indicator of photochemical smog, in which ozone is formed  through chemical reactions involving oxygen, nitrogen oxides and volatile organic compounds in the presence of sunlight. Nitrogen dioxide, a reddish brown gas, contributes the color to the haze you see on high ozone days.  A colorless haze can be formed with just water droplets, but may also contain particulates from dust, soot, and colorless toxic chemicals.

In a nutshell:

Reddish brown haze usually indicates high nitrogen dioxide and high ozone, which are not good to breathe.

Colorless haze can be just fog, but may contain colorless air pollutants.  Best to check pollution reports for more info.

A clear day may be low on ozone but tells you nothing about invisible air toxics.

Here are some good websites if you want to explore further:

Quick facts from San Francisco Bay Area Air Quality Management District:

Ozone information from USEPA for health care providers and patients:

Ozone chemistry for the serious student:

*Ozone pollution is not exactly a milky orange cloud as described in the recent AAH article, “Can You See Air Pollution?”. But you can certainly expect that ozone is in that milky cloud of photochemical smog.