The vast majority of those small, dark, roughly coin-sized spots dotting city sidewalks are pieces of discarded chewing gum that have been flattened by foot traffic and darkened by weathering. What starts as a soft, pale wad stuck to concrete gradually turns into a stubborn black disc that can persist for years. Gum is not the only culprit, though. Lichens, fungal colonies, tree sap, and airborne soot can all leave dark marks on pavement, and telling them apart is easier than you might think.
Why Chewing Gum Turns Black on Concrete
A fresh piece of gum is usually white, pink, or green. Within days of landing on a sidewalk, it begins to flatten under shoes and absorb grime. The gum base itself is a blend of synthetic polymers, resins, and waxes designed to be chewed without dissolving. That same durability means it resists rain, sun, and microbial breakdown once it hits the ground. Ultraviolet light from the sun oxidizes the polymers, darkening them much the way a rubber band left in sunlight turns brittle and discolored. Dirt, oil from shoe soles, vehicle exhaust particles, and general urban grime press into the softened surface. Over weeks, the combination of UV degradation, embedded dirt, and repeated compression turns the gum into a flat, dark circle that bonds tightly to the rough texture of concrete.
The conventional gum base that makes this possible is essentially non-biodegradable. Researchers developing alternatives have described conventional chewing gum, which consists of sweeteners combined with a non-biodegradable gum base, as a source of severe environmental pollution.1PubMed. Design and Synthesis of Biobased, Biodegradable Polyester for Chewing Gum That polymer backbone is what makes each spot so persistent. A piece of natural chewing resin or chicle would eventually soften and wash away, but synthetic polyvinyl acetate and polyethylene compounds resist water, heat, and microbial digestion almost indefinitely under normal outdoor conditions.
How to Tell Gum Spots from Other Dark Marks
If you crouch down and look closely, gum spots tend to share a few distinctive features. They are usually round or oval, roughly the diameter of a coin or slightly larger, reflecting the shape of a wad pressed flat by a shoe. Their edges are fairly well defined, and the surface often has a slightly waxy or rubbery sheen, even after years of exposure. You can sometimes see a faint texture of shoe tread imprinted into the top. A pocketknife or fingernail scraped across the surface will catch on the rubbery material in a way it would not on bare concrete.
Other dark spots have different signatures. Tree sap or fruit droppings tend to form irregular shapes, often clustered directly beneath overhanging branches. They may feel sticky when fresh and glassy when old. Oil drips from parked vehicles usually leave elongated or irregular stains, sometimes with a faint rainbow sheen when wet, and they soak into the concrete rather than sitting on top. Rust stains from metal furniture or fixtures are typically reddish-brown rather than true black. And then there are biological colonizers like lichens and fungi, which behave quite differently from any of these.
Lichens on Sidewalks
Lichens are composite organisms made up of a fungus living in partnership with algae or cyanobacteria. On sidewalks, certain crust-forming species can look like dark or greenish-black spots, and they are sometimes mistaken for old gum or paint splatters. Unlike gum, lichens are alive. They grow slowly, spreading outward in roughly circular patterns, and they bond to the mineral surface of the concrete itself rather than sitting on top of it.
One study of the lichen species Circinaria contorta growing on city sidewalks found that these organisms reach their fastest growth rate at around eight years of sidewalk age, expanding at roughly three and a half millimeters per year at that peak, and eventually leveling off at a maximum diameter of about 62 millimeters.2Lindbergia. Urban lichenometry: growth of Circinaria contorta on city sidewalks That means a mature lichen patch can be bigger than most gum spots and takes many years to reach full size. If you see a dark spot that is larger than a quarter, has a slightly bumpy or crusty texture, and appears to be growing concentrically outward, it is more likely a lichen than discarded gum. Scraping it with a fingernail will produce powdery or gritty fragments rather than rubbery curls.
Lichens prefer surfaces that are left undisturbed, so they tend to colonize the edges of sidewalks, corners near walls, or less-trafficked areas rather than the middle of a busy path. Heavy foot traffic physically disrupts their growth. In that sense, the distribution pattern is the opposite of gum: gum concentrates where people walk, while lichens concentrate where people do not.
Fungal Staining on Concrete
Fungi are another biological source of dark discoloration on sidewalks. Unlike lichens, which sit visibly on the surface, certain microfungi can colonize the pores and surface layer of concrete itself. They produce dark pigments, particularly melanins, as a defense against UV radiation and environmental stress. These pigments stain the concrete from within, leaving marks that cannot simply be scraped off.
Research on microfungi affecting urban concrete has shown that fungal colonization does more than just discolor surfaces. In laboratory experiments, fungal growth on concrete samples reduced the calcium content of the material by roughly 32 to 41 percent, depending on the type of contact.3Hindawi / PubMed Central. Metabolic Activity of Micromycetes Affecting Urban Concrete Constructions Fungi produce organic acids as metabolic byproducts, and these acids dissolve the calcium compounds that give concrete its strength. So fungal dark spots are not just cosmetic; over long periods, they can contribute to surface degradation of the pavement itself.
Fungal staining tends to appear in persistently damp or shaded areas: under awnings where water drips, along the base of north-facing walls, or in spots where soil meets concrete. The marks are usually irregular in shape, sometimes forming diffuse dark patches rather than the neat circles typical of gum or lichens. If a dark area on the sidewalk seems to follow a moisture pattern rather than a foot-traffic pattern, fungi are a likely cause.
Soot and Atmospheric Deposits
In cities with heavy traffic, industrial activity, or frequent construction, fine particulate matter settles on horizontal surfaces and accumulates over time. This is sometimes called atmospheric sooting. The particles are mostly carbon-based, coming from diesel exhaust, tire wear, brake dust, and combustion byproducts. On a smooth surface like glass, soot wipes off easily. On rough concrete, it lodges in the tiny pits and pores of the surface and becomes increasingly difficult to remove.
Sooty deposits tend to create a general darkening of the sidewalk rather than distinct individual spots. You will notice it most clearly where a section of pavement has been recently replaced or power-washed: the clean patch stands out sharply against the surrounding grime. Over years, this gradual darkening forms the baseline against which the discrete spots of gum, lichen, or fungal growth become visible. In a sense, soot provides the gray background, and gum provides the black polka dots.
The Scale of the Gum Problem
Walk down any busy commercial street in a major city and you will notice gum spots numbering in the hundreds per block. London’s Oxford Street became a famous case study in the early 2000s, when surveys counted hundreds of thousands of gum deposits along the shopping thoroughfare. Cities around the world spend significant sums each year on gum removal from public sidewalks, plazas, and transit stations. The cost comes from the labor-intensive process required: because gum bonds so effectively to concrete, removing it typically requires steam, high-pressure washing, specialized solvents, or freezing agents that make the gum brittle enough to chip off.
The adhesive and hydrophobic nature of gum residues, combined with microbial colonization of the gum surface over time, contributes to their remarkable persistence once discarded.4MDPI / Processes. Chewing Gum to Microplastic: Hidden Pollution and Its Scalable Circular Upcycling Pathways Hydrophobic means the gum repels water, so rain does not dissolve or loosen it. Meanwhile, the rough pore structure of concrete acts like Velcro at a microscopic level, giving the flattened gum countless tiny anchor points. Aggressive removal methods like power-washing can damage the concrete surface, leaving a pale ghost mark where the gum was. Some municipalities have moved to lower-impact techniques, including biodegradable cleaning agents and targeted steam applicators, but none are fast or cheap.
Gum Spots and Microplastic Pollution
Beyond the visual blight, discarded gum contributes to the broader problem of microplastic pollution. Chewing gum base contains synthetic polymers, and as those polymers degrade under sunlight and mechanical stress, they shed tiny plastic fragments. Recent research has identified chewing gum as a frequently overlooked source of microplastic-sized particles, both during chewing and after disposal.4MDPI / Processes. Chewing Gum to Microplastic: Hidden Pollution and Its Scalable Circular Upcycling Pathways Early experiments have detected microplastic release into saliva during chewing, with most particles shedding in the first several minutes. On the sidewalk, the same fragmentation process continues as shoes grind the gum and UV light breaks down the polymer chains, releasing particles into stormwater runoff and surrounding soil.
The practical significance of sidewalk gum as a microplastic source relative to larger contributors like tire wear, textile fibers, and packaging waste is still being studied. But the sheer number of gum deposits in any city means the cumulative contribution is not trivial. Each dark spot is essentially a tiny reservoir of synthetic polymer slowly fragmenting into the environment.
How Dark Spots Affect the Feel of a Neighborhood
There is a psychological dimension to all those black dots. Research on how people perceive urban environments has shown that visible litter significantly changes how safe a place feels. In an experimental study with over 600 participants, seeing litter in a filmed environment led people to anticipate more antisocial behavior and to perceive higher crime levels in that area.5Elsevier. Litter, gender and brand: The anticipation of incivilities and perceptions of crime prevalence While that study focused on litter in general rather than gum spots specifically, the mechanism is the same: visible signs of neglect signal that an area is not being maintained, which triggers assumptions about disorder and safety.
This is part of why cities invest in gum removal even when the material is technically harmless to walk on. A sidewalk covered in dark spots reads as dirty and neglected, which affects foot traffic, property values, and the willingness of people to linger in commercial areas. The spots are small individually, but collectively they change the character of a streetscape. Business improvement districts and transit authorities often treat gum removal as a core maintenance task for exactly this reason.
Biodegradable Gum and Non-Stick Formulas
The persistence of gum on sidewalks has motivated a line of research into gum bases that break down after disposal. Several groups have developed formulations using poly(lactic acid), a biodegradable polyester derived from plant starches, as a replacement for conventional synthetic gum base. One research team created a biodegradable chewing gum using a blend of plasticized poly(lactic acid) and Pistacia atlantica gum, reporting that the resulting product showed promising physical properties and genuine biodegradability while maintaining acceptable chewing texture.6PubMed Central. Development of Biodegradable Chewing Gum Using Plasticized Poly(Lactic Acid)/Pistacia atlantica Gum Blend: A Novel Gum Base Elastomer for Sustainable Formulation
Other researchers have explored similar poly(lactic acid) bases, finding that certain plasticizers allow the gum to biodegrade considerably after disposal while still delivering acceptable hardness, cohesiveness, and chewiness during use.7Tropical Journal of Pharmaceutical Research. Formulation and characterization of caffeine biodegradable chewing gum delivery system for alertness using plasticized poly (D,L-lactic acid) as gum base The idea is straightforward: if gum base breaks down when exposed to moisture and microbes in the environment, discarded pieces would soften, fragment, and eventually disappear rather than becoming permanent black spots. A few commercial products using non-stick or biodegradable formulations have reached the market in recent years, though they remain niche compared to conventional gum.
The challenge is that the very properties that make conventional gum satisfying to chew, its elasticity, moisture resistance, and durability, are the same properties that make it stick to sidewalks and refuse to break down. Engineering a material that holds up during 20 minutes of vigorous chewing but then degrades rapidly once spat onto wet concrete is a genuinely difficult materials science problem. Progress is real, but the black spots are not going away soon.
A Quick Field Guide
If you are curious enough to investigate the dark spots on your local sidewalk, here are the distinguishing features of each common type:
- Chewing gum: Coin-sized, round or oval, slightly raised or flush with the surface, rubbery when scraped, concentrated in high-foot-traffic zones like crosswalks and bus stops.
- Lichen: Crusty or bumpy texture, grows in a roughly circular pattern, often larger than gum spots at maturity, powdery when scraped, found in low-traffic areas near walls or edges.
- Fungal staining: Irregular and diffuse rather than sharply circular, concentrated in damp or shaded spots, embedded in the concrete surface rather than sitting on top, sometimes accompanied by visible surface pitting.
- Tree sap or fruit: Irregular shapes clustered beneath branches, sometimes sticky or glossy, may attract insects or have a faintly sweet smell.
- Oil drips: Elongated or irregular, often near curbs or parking spots, may show a faint rainbow sheen when wet, soaks into concrete rather than sitting on the surface.
- Soot: General darkening rather than discrete spots, most visible by contrast when adjacent pavement has been cleaned or replaced.
The distribution pattern alone tells you a lot. Gum clusters where people wait, stand, and walk. Lichens and fungi cluster where moisture lingers and feet do not. Oil and sap follow their sources. And soot covers everything equally, like a slow urban patina that takes years to notice unless someone power-washes a comparison patch into the concrete.