= Solution
\b[Distinguish diseases caused by inhaled <air pollutants> from <infections> transmitted through air. The principal impurity groups are gases and vapours, suspended particles and biological material; detection must identify the particular group and, where possible, measure its concentration.]
The nineteenth-century phrase “impure air” mixed several different mechanisms. Irritant smoke and dust can damage the <respiratory system>; toxic gases can poison without causing an <infection>; crowded, poorly ventilated rooms can increase exposure to infectious material. The historical <miasma theory> treated foul emanations much more broadly as generators of disease. In particular, the smell of sewage does not itself establish the cause of <typhoid fever>, <cholera> or <malaria>.
For the first part, important real consequences include the following:
* <Particulate matter> and irritant gases aggravate <asthma> and contribute to chronic <respiratory disease>. Fine <particulate matter> is also associated causally with <cardiovascular disease> and <lung cancer>; these are modern conclusions, not a list of mechanisms all established in 1867.<Ambient outdoor air pollution by the World Health Organization>[“Overview” and “Health effects”: particulate exposure, cardiovascular and respiratory disease and cancer]{ref}
* <Carbon monoxide> from incomplete <combustion> can cause <carbon monoxide poisoning>, with headache, weakness, confusion and potentially fatal impairment of <oxygen> delivery. Accumulated <carbon dioxide> and reduced <oxygen> in confined spaces present different hazards; a high <carbon dioxide> reading does not identify every other contaminant.<Carbon monoxide poisoning basics by the Centers for Disease Control and Prevention>[“About carbon monoxide”: combustion sources, symptoms and severe effects]{ref}
* <Sulfur dioxide>, <Ammonia> and <hydrogen sulfide> can injure the eyes and <respiratory system>; sufficiently high <hydrogen sulfide> exposure can rapidly incapacitate or kill. Industrial dusts create additional substance-specific hazards, so “dust” is not one chemically uniform <poison>.<Hydrogen sulfide by the National Institute for Occupational Safety and Health>[Pocket Guide, “Symptoms” and “Target Organs”: eye and respiratory irritation, central effects and respiratory arrest]{ref}
* Airborne infectious material can transmit particular <infections>, including <tuberculosis>. Ventilation changes dilution and removal; it does not create or abolish a pathogen's identity. This is a different process from <carbon monoxide poisoning> or irritation by smoke.<Tuberculosis: Causes and How It Spreads by the Centers for Disease Control and Prevention>[“How it spreads”: airborne material from infectious pulmonary or throat disease, especially indoors with poor air circulation]{ref}
For the second part, ordinary outdoor air already contains <nitrogen>, <oxygen>, a small amount of <carbon dioxide>, and variable <water vapour>. Their presence alone is not evidence of pollution. In a smoky industrial town, <soot>, mineral dust, products of fuel <combustion> and <sulfur dioxide> would be particularly relevant. In occupied rooms, expired <carbon dioxide>, <water vapour> and suspended biological material increase unless removed. Decomposing wastes may contribute <Ammonia>, <hydrogen sulfide> and other volatile products. The actual mixture depends on sources, ventilation and weather.
For the third part, begin with a defined sampling site and time, record occupancy or industrial activity, and compare with a suitable reference. A volume of air, not simply an unrecorded sniff or a discoloured paper, is the basis of quantitative comparison. Methods available by 1867 include:
* \b[<Carbon dioxide>: absorption followed by measurement of the reagent consumed.] In the <baryta absorption method>, expose a known volume of air to a measured amount of standardized <barium hydroxide> solution. <Carbon dioxide> forms <barium carbonate>; compare the residual <alkalinity> with a reagent blank by <titration>. A contemporary Manchester investigation used 7–10 litre globes for this approach and checked it against gravimetric absorption in potash. This is a useful example of two different measurement principles checking each other.<Chemical News volume 9 (1864)>[report of the Manchester Literary and Philosophical Society, “Note on the Amount of Carbonic Acid contained in the Air of Manchester”: globe method and independent weight determinations]{ref}
* \b[Sulfur-containing gases: use chemically distinct tests.] <Hydrogen sulfide> blackens paper moistened with a soluble <lead> salt by forming <lead sulfide>. Acidic <sulfur dioxide> changes moist indicator paper and has reducing properties; a coloured reagent's loss of colour needs controls because other reducing substances interfere. A positive result establishes a reaction, not by itself a measured safe or dangerous concentration.
* \b[<Ammonia>: capture and demonstrate alkalinity or a characteristic chemical reaction.] Draw air through clean <water> or a suitable absorbing liquid, test the collected material and compare it with an unexposed blank. Moist red <litmus> turning blue is a simple indication of an alkaline gas, but cannot by itself distinguish <Ammonia> from every other base.
* \b[Suspended material: collect before examining.] Deposit particles on a clean surface or capture them by filtration, then use <microscopy> to distinguish obvious <soot>, fibres, mineral particles, pollen and other material. A measured volume and collected mass permit a dust burden to be estimated. Visible particles are not a direct measurement of infectiousness, and a nineteenth-century microscopic inspection cannot exclude all pathogenic material.
* \b[<Carbon monoxide>: recognise the limitation of smell and generic air tests.] It is colourless and odourless. Its interaction with <hemoglobin> provides a basis for chemical and spectral investigation, while modern calibrated instruments permit direct monitoring. An acceptable <carbon dioxide> result or a lack of offensive smell does not rule it out.
The chemical equations underlying two of these qualitative tests are
$$
\mathrm{Ba(OH)_2+CO_2\longrightarrow BaCO_3\downarrow+H_2O},
\qquad
\mathrm{Pb^{2+}+H_2S\longrightarrow PbS\downarrow+2H^+}.
$$
Here the precipitates are <barium carbonate> and <lead sulfide>. For the <baryta absorption method>, in an ideal, fully captured sample with no interfering acid gases, the difference between blank and sample acid requirements satisfies
$$
\boxed{n(\mathrm{CO_2})=\frac{c_{\mathrm{H^+}}(V_{\mathrm{blank}}-V_{\mathrm{sample}})}{2}},
$$
where $c_{\mathrm{H^+}}$ is the amount concentration of titratable acid equivalents and the volumes refer to titration of the entire corresponding absorbent portions; any aliquot requires its scale factor. The factor two follows from the two acid equivalents neutralized per <mole> of <barium hydroxide>. This calculation assumes that the <barium carbonate> precipitate is excluded from the residual-base determination; dissolving it again with the titrant would invalidate the inference.
Odour, visible smoke and room stuffiness are useful prompts to investigate, but \b[no single smell, <carbon dioxide> value, microscopic field or chemical colour test establishes that air is free of harmful impurities]. Specify which material was measured and which uncertainty remains.
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