“Exposure Density” and the Pandemic


A week ago, I posted Early Observations on the Pandemic and Population Density, which suggested that the more worrying experience with the COVID-19 virus in the New York City metropolitan area could result from more intense person-to-person contacts:

…the risk of infection is a function of being close to people who are infected.The most fundamental issue is thus, how close people are to one-another in their daily lives. The risk of infection can be expected to be higher where there are very high densities whether in residences, transport or employment locations.

Jason Fox of Bloomberg, countered that weighted “metropolitan area” densities did not support my observations. It is true that the weighted metropolitan density of New York is only a bit more than double that of San Francisco and Los Angeles. He could have made his point even stronger using US Census Bureau urban area densities showing Los Angeles to be the densest, (2,700 per square kilometer or 7,000 per square mile, San Francisco second (2,400/6,300) and San Jose to be denser than the New York urban area (2,100/5,300). However this was not the subject of "Early Observations", which dealt with density at the personal level.

Exposure Density and COVID Infection

Person to person contact might be described by a term that has been used in medicine, especially radiology, “exposure density.” Greater exposure density likely lead to greater infection risk for people with a higher density of personal contacts over a greater period of time, all else equal.

Exposure density for an illness spread person to person and for which an important preventive strategy is “social distancing” is a function of densities at a micro level. Exposure density, and thus infection, is likely to be less, all else equal, if common halls, elevators, crowded places and transportation facilities are avoided.

Update on the Situation

According to the Johns Hopkins virus dashboard, as of 11:45 A.M. EDT April 10, New York City, with, less than 3% of the national population, accounted for 19% of the COVID-19 infections and 31% (over 5,000) of the deaths. Sixteen of the 25 counties with the largest death tolls (the City is counted as a county) stretch along the commuter rail lines from New York’s two main stations (Grand Central Station and Penn Station). This includes Nassau County, on Long Island, which has the second highest toll. Suffolk County, also on Long Island has the fourth largest death count, Westchester County, bordering the City on the North ranks fifth, while Bergen and Essex counties in New Jersey have the seventh and eighth highest counts.

The largest death counts also include Los Angeles County, the nation’s most populous county, which has more people than New York City, but far fewer deaths, around 200, as well as the second largest, Cook County (Chicago), with about 350 and a population more than 60% as large as New York City.

The Uniqueness of New York City

Simply put nowhere in America has remotely the micro-densities of New York City whether in terms of residential and employment locations or most other places that people congregate. This distinguishes New York’s urban core from other important ones such as Chicago, Philadelphia, Boston and San Francisco. These may think they are little “New Yorks” but they are simply not in the same league.

In New York City, two-thirds of the population (5.4 million) people live at densities above 15,000 per square kilometer. This is 80% of the national total.

For example the nation’s second densest large municipality (after New York City), San Francisco, only 6,200 people (0.006 million) live at densities above 15,000 per square kilometer. This small population is only 0.001% of the national total at 15,000 per square kilometer or higher. If San Francisco had the same percentage of its population living at this density as New York City, it would be home to 2.5 million residents, not the under 900,000 who live there.

But if New York’s residential densities dwarf all others in the nation, its employment densities are even more stratospheric — and arguably more dangerous in terms of “exposure density” — by comparison. The highest employment density census tract is 322,000 per square kilometer (834,000 per square mile). New York dominates CBD employment data even more than it does residential densities. By contrast, the average urban employment density in the United States is about 600 per square kilometer (1,000 per square mile).

But New York City is also just about as different as much of the New York City metropolitan area. More than one-half of the metropolitan area’s jobs are outside the city of New York according to the 2018 American Community Survey. Most people in the suburbs have a lifestyle more like that of suburban Los Angeles, Dallas-Fort Worth or Atlanta. More than 65% live in ground-oriented housing (detached, semi-detached and townhouses). This compares to around 15% in the City. Transit commuting within the suburbs is similar to that of the rest of the country, in contrast to transit’s Manhattan-centric pattern

Commuting in the New York Metropolitan Area

None of this means, however, that all New York metropolitan area residents and workers are at anything like equal exposure risk.For example:

  • A commuter from a detached house in Westchester County to a job in Lower Manhattan’s financial district is likely to experience high exposure density. The trip, for the sake of discussion, includes a walk or car to the commuter rail station, a ride on commuter rail to Grand Central Station, a walk through corridors to reach the subway station, a ride on the subway train, exiting through a subway station for a walk to the high rise work location. At this point, the commuter joins others in a crowded elevator, and exit at the 40th floor, walking to an office shared with others.
  • By contrast, the commuter in the next-door Westchester County detached house works on Interstate 287 corridor in New Jersey. Driving alone, the commuter reaches work while experiencing minimal exposure density. The work location is in a low rise building without an elevator. The only place the I-287 commuter encounters any sort of exposure density is in the actual work environment. Even this is likely to be considerably less dense than the New York financial district office.
  • Other residents, who live in high rise residential buildings are likely to experience greater exposure densities because they must use common hallways and elevators. One New York developer expressed concern about the high-density residential market, calling the City as “a gargantuan petri dish.” The generally small and often crowded New York City neighborhood markets, where many people do so much of their shopping, probably have higher exposure densities than the larger stores typical in suburban areas with their parking lots and wide aisles.

In short, the social distancing required to minimize the spread of an infectious disease such as COVID-19 is much more difficult in New York City and in the densely packed mass transit corridors leading to it than in most other areas of the nation.


The most useful research, after this nightmare has passed, will assess the situation and behavior of those infected and who have died.Exposure density, which will include other factors than those above, is likely to explain much. Like last week, all of the observations suggested above are preliminary.

Note: It is to be hoped that COVID-19 research will avoid the all too frequent use of metropolitan area densities as a surrogate for urban densities. With 97% of metropolitan area land being rural, use of metropolitan density data for this purpose is inherently inadvisable. Fortunately, the US Census Bureau has been producing urban area population and density data since 1950, and this should be used for any research requiring urban densities.

Photograph: Outer New York suburbs (by author)

Wendell Cox is principal of Demographia, an international public policy and demographics firm. He is a Senior Fellow of the Center for Opportunity Urbanism (US), Senior Fellow for Housing Affordability and Municipal Policy for the Frontier Centre for Public Policy (Canada), and a member of the Board of Advisors of the Center for Demographics and Policy at Chapman University (California). He is co-author of the "Demographia International Housing Affordability Survey" and author of "Demographia World Urban Areas" and "War on the Dream: How Anti-Sprawl Policy Threatens the Quality of Life." He was appointed by Mayor Tom Bradley to three terms on the Los Angeles County Transportation Commission, where he served with the leading city and county leadership as the only non-elected member. Speaker of the House of Representatives appointed him to the Amtrak Reform Council. He served as a visiting professor at the Conservatoire National des Arts et Metiers, a national university in Paris.