Of the three aspects of the Brexit divorce the EU says must be tackled before they will move onto the future relationship, the rights of EU citizens living in the U.K. and the reciprocal rights of British citizens across the Continent was supposed to be the easy one. (The other two are the Northern Irish border and the so-called Brexit bill.)

Both sides say they want a quick win on the issue to give certainty to the millions of people now concerned about their immigration status. But Prime Minister Theresa May's offer of "settled status" to EU citizens living in the U.K. has not gone down as well as she hoped and the topic is set to take up a considerable amount of oxygen in the first full week of Brexit talks, which get under way in Brussels Monday.

There are around 3.2 million EU citizens living in the U.K. and around a million U.K. citizens resident in the rest of the bloc. But who are they? And what impact do they have on the countries they live in? Here's POLITICO's data guide to the issue:

Where do they come from?

In many of the following metrics, there are big differences between citizens of the EU14 (Western European countries that joined the EU before 2002) and citizens of the EU8 and EU2 (newer member countries, mostly from Eastern Europe). Malta and Cyprus also joined the EU in 2004, but are not included in figures from the Office for National Statistics because they already enjoyed rights in Britain as members of the Commonwealth.

Here's how many citizens of each country of the rest of the EU live in the United Kingdom.

How old are they?

Most EU nationals who come to the U.K. for work are young; more than half of Britain's non-U.K. nationals of working age were between 25 and 39 years old.

Britain has an aging population — by 2040, nearly one in seven British people will be over 75. The loss of these young workers will likely mean British citizens having to retire later.

Sarah Harper, the director of the Oxford Institute of Population Aging and chair of the U.K. government’s foresight review on aging societies has warned about this problem. “The message from Brexit is if you don’t want immigrants, you’re going to have to work longer," Harper told the Guardian in January. "That’s how the sums work.”

One striking statistic: there are 88 people of working age from the EU8 group of Central and Eastern European nations living in the U.K. for every one person over the age of 65 from the same group. The ratio is far higher than for U.K. citizens. For Brits, the equivalent “dependency ratio” is 3.5 people of working age — defined as between 15 and 65 — for every one person older than that.

When did they arrive?

Most EU citizens living in Britain arrived in the last 15 years — not a surprise given that freedom of movement wasn't an option for many EU countries until they joined the bloc, or for years after.

The second largest segment is people who have been in the U.K. for more than 30 years — more than 100,000 EU8 citizens have been in the country since before 1981.

What do they do?

Last year, 11 percent of the U.K. workforce was non-U.K. nationals, with 7 percent from the EU27 — roughly 2.2 million people. Both the EU14 and the EU8/EU2 populations have higher rates of employment than United Kingdom citizens.

Where do they work?

Many sectors of the economy depend significantly on EU workers. Manufacturing, construction and agriculture in particular rely on the work of immigrants from the EU8 and the EU2, who are often willing to accept lower-paid jobs that U.K.-born workers are less keen to do.

In reality, the agriculture sector is far more dependent on the work of foreigners than current data demonstrates, due to seasonal farm labor that is not included in these figures. The U.K. needs about 80,000 migrant farm laborers each year to pick its crops, and this sector is facing repercussions from the Brexit vote: A new National Farmers' Union survey found there was a shortfall of workers of nearly 20 percent in May, and the drop has been blamed on a perception of the U.K. as xenophobic and unfriendly toward foreigners.

The NFU survey found that three-quarters of the farm laborers recruited in the first five months of 2017 were from Bulgaria and Romania, and almost all of the rest from Eastern Europe. Just 14 of those workers were British.

Low- or high-skilled?

Counterintuitively, as the agriculture sector demonstrates, the low skill-level job sector is more dependent on foreign workers than the high skill-level sector. Though most people from the EU14 who work in the U.K. are employed in highly-skilled areas, they make up a smaller part of the skilled set of workers in the U.K. than foreigners do of the lower skill-level sector. Nearly one in five low-skilled workers in Britain are non-U.K. nationals.

Is London different?

London has the highest concentration of any region of highly-skilled non-U.K. citizens, mostly working in financial services or the public administration/education/health sectors. Nine percent of skilled workers in London are not U.K. nationals.

What do they earn?

Many immigrants in the U.K. are over-qualified for the roles they do (40 percent of the EU8 nationals working in the U.K. are over-educated for their jobs). Immigrants from the EU8 and EU2 are paid less and work more hours on average than U.K. nationals.

Is the UK still an attractive destination?

A survey of skilled workers in the U.K., conducted by law firm Baker McKenzie, found that more than half of EU nationals in the country were likely to leave the U.K. before Brexit negotiations had finished. Most of them feel more vulnerable to discrimination and less secure in their jobs than before the Brexit vote.

“The strength of feeling was surprising given the length of time after the referendum,” Baker McKenzie's Stephen Ratcliffe said. “There are a lot of people in those sessions [during which the data was collected] who were really upset, and they said ‘Well, if you don’t want me here, I’m going.’”

And it's not just workers leaving that changed after the Brexit vote — workers are now less likely to even come to the U.K. as well.

In particular, immigration from the EU8 declined drastically after the Brexit referendum in June 2016. This group makes up large swaths of the agriculture and manufacturing sectors (7 and 8 percent respectively).

Immigration was cited as one of the main reasons the U.K. voted for Brexit. EU citizens have already started to take the hint.

What do they contribute to the economy?

Because they are generally younger and more likely to be in work than U.K. citizens, EU citizens make a significant contribution to the U.K. exchequer. According to the government, they paid £3.11 billion in income tax and national insurance contributions in the 2013-2014 tax year (the most recent figures) and received £560 million in benefits.

How much does their health care cost?

Under reciprocal arrangements with other EU members, Britain pays for the health care of U.K. citizens living across the Continent and receives equivalent funds for EU citizens living in the U.K. The care costs for British ex-pats are far higher because they are much more likely to be retirees.

How much do pensions for EU citizens cost?

You don't have to be British to qualify for a British pension — you just have to have contributed enough to the country's national insurance scheme. As such, there are British pensions being paid to residents of other countries.

There has been a dramatic increase in the number of recipients of a U.K. pension in Poland in particular. The U.K.'s Office for National Statistics says the increase is likely due to EU8 migrants returning to their home country and claiming the state pension they are eligible for from working in the U.K.

Where are U.K. citizens across the Continent living?

The other side of the coin in the Brexit talks are the million or so U.K. citizens residing across the rest of the EU bloc. Here's where they are living.